Welcome to the Mineral and precious stone glossary

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Abrasive

Hard, tough material used to smooth out rough surfaces; a common abrasive is sandpaper.

Absolute hardness

Scale for measuring the hardness of a mineral. The Mohs scale also measures hardness, except the absolute hardness scale has its numbers in proportion. Only scientists use the absolute hardness scale; mineral collectors measure hardness using the Mohs scale.

Absorption spectrum

The specific bands of light that pass through and those that get absorbed in a gem or mineral and disperse into the colors of the spectrum. Different gems have different absorption spectra, meaning they are all unique in regard to which colors are absorbed and which pass through. This plays a major role in identifying gems, and can easily distinguish between an imitation gem that has properties almost identical to the real thing. A spectroscope is used to determine the absorption spectra of a mineral.

Accessory mineral

Mineral that occurs with other minerals.

Acicular

An aggregate of long, slender, crystals (i.e. Natrolite). This term is also used to describe the crystal habit of single long, thin, slender crystals.

Acid

Any of a group of chemicals containing a free hydrogen element. Certain acids are used to clean minerals, and tests can be performed on some minerals with certain acids. See also acid test, hydrochloric acid, and nitric acid.

Acid test

Procedure performed to help identify a mineral. Certain acids (usually hydrochloric acid and nitric acid) are placed in contact with the mineral or its powder, and, depending on the mineral, it dissolves, effervesces, or remains inactive.

Activator

Particle in a fluorescent substance that causes the substance to glow when exposed to ultraviolet light.

Adamantine (luster)

Transparent minerals with a very high luster are said to have an adamantine luster. (Translucent to nearly opaque minerals with a very high luster are said to have a submetallic luster).

Adularescence

An effect seen on certain minerals which causes it to display a billowy, rounded, ghost-like reflection with a bluish-whitish color emanating from the surface when the mineral is cut into a cabochon. It is caused by structural anomalies or build up of water in the mineral. The minerals most famous for exhibiting adularescence are Opal and Moonstone, and the name is derived from Adularia, of which Moonstone is a variety of.

Aggregate

A grouping of crystals. Aggregates are defined by the ways crystals are clustered together. (See Crystals in Mineral Properties for more information.)

Alkaline

Describing a chemical substance that is either a hydroxide, carbonate, or metal oxide that has the ability to turn litmus paper blue, and the ability to react with acids to form salts.

Alloy

A mixture of two or more metals or semi-metals in which the atoms of the different metals are intermingled among each other. Some minerals, such as Josephinite (Fe,Ni), are naturally occurring alloys.

Alloyed

Mixed with another metal to form an alloy.

Alluvial

Said of eroded material that gets transported by rivers or streams.

Alluvial deposit

Deposit in a stream or river where alluvial materials get placed .

Alluvium

Eroded material that gets carried downstream by the current of the stream or river. Alluvial deposits are areas in streams or rivers where alluvium does not continue flowing downstream, thus forming a deposit.

Alpha rays

Form of electromagnetic radiation in which the electromagnetic waves are composed of helium atoms with a double positive charge (He2++). Alpha rays have very long wavelengths. See also beta and gamma rays.

Amalgam

Any alloy of mercury and another metal. Some amalgams occur naturally. Note: There is a mineral known as Amalgam, and, although it is an amalgam, should not be confused with the term amalgam.

Amber

Fossilized pine resin, many times used for ornamental purposes.

Ammonia

A colorless, reeking gas with a chemical formula of NH3. When combined with the hydroxyl radical (OH) it forms the alkaline liquid ammonium hydroxide, which is the common household cleaner called "ammonia" (even though it is ammonium hydroxide).

Ammonium hydroxide

Basic liquid which has many household uses and can be used to test some minerals. Ammonium hydroxide has a chemical formula of NH4(OH), and is usually incorrectly called "ammonia", which is a gas similar to Ammonium hydroxide but lacking water.

Amorphous

Without a crystalline shape.

Amphibole

Individual member of the amphibole group.

Amphibole group

Group of minerals that contain iron, magnesium, silicon, oxygen, and hydroxyl (OH). They may also contain calcium, sodium, and lithium. The amphiboles have prismatic cleavage, and one can note the angle of 56° and 124°, which distinguishes it pyroxenes, which are at 87° and 93° (almost perpendicular). The amphiboles are very similar to the pyroxenes, and the minerals belonging to each group many times cannot be distinguished by ordinary methods. Generally, though, the amphiboles occur in longer, thinner crystals than the pyroxenes. The pyroxenes contain the same elements that the amphiboles do, except they lack hydroxyl, which alters both physical and chemical properties. The amphibole group is in the inosilicate subdivision of the silicates group.

Amygdule

A small, bubble-like mass formed from volcanic igneous rock that solidified and a bubble of gas got trapped in the interior.

Anhydrous

Without water. Anhydrous minerals contain no water in their chemical structure. The term anhydrous is usually used in reference to an anhydrous mineral belonging to a group which contains many hydrous minerals.

Anion

Neutral atom that gains an electron and becomes negatively charged.

Anisotropic

Lacking consistent hardness on all surfaces. For example, the hardness of Kyanite on the Mohs scale is between 6 and 7 lengthwise, and between 4 and 4½ crosswise.

Anticline

Fold in rock where the strata protrudes upwards towards the fold, and each section of rock reaches its highest point by the fold area.

Antimonides (subgroup)

Group of sulfides that contain one or more true metals combined with the semi-metal antimony.

Aqua regia

(HCl),(HNO3) Mixture of hydrochloric and nitric acids. It is an extremely destructive mixture and can dissolve gold and platinum, as well as many other minerals.

Aqueous

Formed from precipitating hard water. Stalagmites and stalactites are common examples.

Aragonite group

Group of minerals belonging to the carbonate group that are isomorphous with one another. They all crystallize in the orthorhombic system, exhibit good cleavage (although not as good as the members of the calcite group), have a weak double refraction in transparent specimens, and commonly intergrow in three individuals forming six sided trillings.

Argentiferous

Containing silver.

Argillacious

Composed mostly of clay.

Arid

A dry, desert region. Many minerals that exist only in arid regions are usually the result of evaporation.

Arsenates

Group of minerals that are compounds of one or more metallic elements associated with the arsenate radical (AsO4). The arsenates, together with the related vanadates, are classified in the phosphate group. Most arsenates are heavy, and none are hard. They are usually brittle and occur in small crystals or compact aggregates.

Arsenides (subgroup)

Group of sulfides that contain one or more true metals combined with the semi-metal arsenic.

Asbestos

Fibrous minerals of the amphibole group, as well as fibrous Serpentine, are known as asbestos. See Actinolite, Tremolite, or Serpentine for more information.

Asbestos amphibole

Any extremely fibrous mineral of the amphibole group. See asbestos; Actinolite / Tremolite.

Asterism

Effect exhibited on some minerals (usually only in polished cabochons) causing it to reflect a billowy, star-like formation of concentrated light which moves around when the mineral is rotated. Asterism is caused by dense inclusions of tiny, parallel, slender, fibers in the mineral which cause the light to reflect in such an interesting manner. Minerals that display asterism may exhibit four, six, and sometimes twelve rayed "stars", depending on the inclusions, size, and facet mode. Some specimens may display much stronger asterism than others, and some specimens may have areas where the inclusions are not present, leaving holes or empty areas in the star.

Asteroid

Large solid mass suspended in outer space that revolves around the sun. Asteroids are larger than meteoroids. See also meteor.

Asymmetrical

Not containing perfect symmetry.

Atom

The structure of elements. Everything is made up of molecules, which are organized clusters of atoms.

Auriferous

Containing gold.

Autochthonous

Describing a rock or mineral that formed in the same location where it was found.

Banded

Forming "bands" of different colors. See banding. Also refers to wavy bands of veins in surrounding rock.

Banding

The presence color zoning lines, or "bands", in some minerals.

Basal

Having to do with the base (basal pinicoid).

Basal cleavage

Type of cleavage exhibited on a horizontal plane of a mineral by way of its base. Minerals with basal cleavage can sometimes be "peeled". Example = minerals of Mica group.

Basalt

Volcanic rock.

Base

1) The top and the bottom "closing points" of a crystal. A base exists only in minerals that belong to the hexagonal, orthorhombic, tetragonal, and trigonal crystal systems. Also known as basal pinicoid. 2) Matrix or foundation of a crystal. 3) Chemical substance that is either a hydroxide, carbonate, or metal oxide that has the ability to turn litmus paper blue, and the ability to react with acids to form salts. Bases are alkaline and have a bitter taste.

Batholith

Enormous mass of igneous rock that intruded through a layer of sedimentary rock at great depths.

Bead test

Complex, scientific test which is conducted to identify a mineral. A mineral is crushed and mixed into a borax flux, and is heated until a glassy bead forms. The bead is then touched by the crushed mineral powder and one of several colors appears on the bead, depending on the metallic elements of the mineral. The colors are different in most cases when heated with an oxidizing and reducing flame, as well as when the bead is hot and cold.

Bed

1) Rock mass of one type surrounded by a different type of rock. 2) A deposit of granular rock caused from erosion of solid rock.

Bedding

The horizontal layers of sedimentary rock still unchanged since the sedimentation process.

Bedrock

Layer of solid rock underneath the soil.

Beta rays

Form of electromagnetic radiation in which the electromagnetic waves are composed of uncombined electrons. See also alpha and gamma waves.

Bituminous

Consisting of hydrocarbons (such as coal) and materials formed from them.

Bladed

Crystal habit describing flat, elongated, "knife-like" crystals. (i.e. Kyanite)

Blowpipe test

Complex, scientific test which is conducted to identify a mineral. A mineral fragment is placed in a cavity on a charcoal block, and a horizontal flame is sent towards the fragment on the charcoal using a blowpipe. Depending on the mineral, a different reaction takes place. Certain metals have a characteristic color when flamed, and if the mineral being tested contains such a metal, the charcoal turns that color behind the fragment. Some minerals form a metal bead after being flamed, others give off fumes, and a few volatilize.

Boiling point

he temperature that it takes a substance to start boiling after being in a liquid state.

Bond

The link between atoms to form a secure molecule.

Borates

Group of minerals that are compounds of one or more metallic elements combined with the borate radical (B2O3). This group forms two sub-groups, the Hydrous borates and the Anhydrous borates. (See "All about borates group")

Borax

Industrially used chemical which is an anhydrous form of sodium borate (Na2B4O7) used in the manufacture of glass and ceramics. It is interesting to note that there is a mineral Borax (Na2B4O7 · 10H2O), and it is different than the borax chemical, for it contains water. The mineral Borax is an ore of the chemical borax. See also Kernite.

Botryoidal

Aggregate resembling a cluster of grapes. Also known as globular. Rounded agglomerations of botryoidal aggregates are smaller than reniform agglomerations and considerably smaller than mammilary agglomerations.

Boule

Synthetic gems created from molten liquids placed in tear shaped molds to crystallize, leaving them with a tear-like form. Mostly applied to synthetic Rubies and Sapphires.

Brass

Man-made alloy of copper and zinc. In the olden times, any alloy of copper was known as brass. Most brass of the olden days was an alloy of copper and tin. See also bronze.

Brittle

Form of tenacity which describes a mineral that gets hammered and results in a fine powder or small crumbs. Minerals that are not brittle are referred as Nonbrittle minerals. Brittle minerals leave a fine powder if scratched, which is the way to test a mineral to see if it is brittle. (See Tenacity in mineral properties for more information.)

Bronze

"Bronze" is a collective term describing any metallic alloy that range in color from light yellow to orange. Many, if not most alloys described as "bronze" are of copper and tin. See also brass.

Cabochon

Gem without facets that is highly polished and has smooth, rounded edges.

Calcareous

Containing the compound calcium carbonate.

Calcic

Containing calcium.

Calcite group

Group of minerals belonging to the carbonate group that are isomorphous with one another and have the same properties, such as that they all:

• Crystallize in the trigonal sect of the hexagonal system, and most commonly form rhombohedrons and scalenohedrons.
• Have perfect rhombohedral cleavage
• Exhibit a very strong double refraction in transparent rhombohedrons

Minerals in this group can partially or fully replace one another. For example, iron may take the place of magnesium in Magnesite (MgCO3), transforming it to Siderite (FeCO3).

Cameo

Gem with a design or figure carved out of the stone, and raised above the background layer.

Capillary

Aggregate composed of tiny, thin, straight, long crystal strands; hairlike. (i.e.Millerite)

Carat

Weight measurement used in reference to gemstones in regard to their evaluation. A carat is .2 grams (or 200 milligrams), and this weight is used worldwide, even in the U.S. where the metric system isn't used. A point is the weight used only in reference to very small, precious gemstones, and represents 1/100th of a carat. The abbreviation for carat is Ct. and for point is Pt. The term carat in regard to gemstones should not be confused with the term carat in regard to gold. By gold, it refers to the content of gold a gold ornament contains. Because of the confusion, the term carat in regard to gold has been changed to karat. (See karat)

Carbonates (group)

Group of minerals that contain one or more metallic elements plus the carbonate radical (CO3). Most are lightly colored and transparent when pure. All carbonates are soft, brittle, and effervesce when exposed to warm hydrochloric acid.
The carbonates are divided as follows:
Calcite Group
Aragonite Group
The Nitrates and Borates are sometimes considered a category of the carbonates.

Carlsbad twin

Form of penetration twinning where two Orthoclase crystals form interpenetrating twins

Carving

Ornamental figure, such as a stone lion, carved out of a rock or mineral.

Cation

Neutral atom that loses an electron and becomes positively charged.

Cat's eye

A mineral with dense inclusions of tiny, parallel, slender, fibers that may cause it to exhibit chatoyancy. The most notable cat's eye mineral is Chrysoberyl Cat's Eye, which is known simply as Cat's Eye. Other cat's eye minerals are termed by their respective names, such as a cat's eye Quartz is known as "Quartz Cat's Eye".

Cave

Large hollow cavity in the earth surrounded by a carbonate or sulfate composed material (usually Calcite) that has an opening to the surface. Caves develop through underground streams carving out the underground minerals.

Cavity

Hollow area in rock that develops because of some form of stress. Many cavities are lined with crystals. (See vug)

Cementation zone

Underground area where certain elements are concentrated and can combine to form new, or primary, minerals.

Center of symmetry

Central area on a polyhedron where all the planes of symmetry intersect.

Chatoyancy

Phenomenon of certain cat's eye minerals which causes it to exhibit a concentrated narrow band of reflected light across the center of the mineral. Chatoyancy is usually only seen on polished cabochons.

Chemical

A substance with a distinct molecular formation, produced by a chemical process.

Chemical bond

A force by which atoms are bound in a molecule or crystal.

Chemical formula

The scientific method of describing what elements a material is composed of. See The Chemical Properties of Minerals for more information.

Chemistry

The study and science of the composition and structure of all substances.

Chemist

Individual practicing in the subject of chemistry.

Chromates (group)

Group of minerals that are compounds of one or more metallic elements combined with the chromate radical (CrO4). Minerals in this group are usually brightly colored and heavy. The chromates are rare minerals and are usually classified as a sub-group of the sulfates.

Chrysoberyl twin

Form of contact twinning, in which six Chrysoberyl crystals join at the base, forming a six-pointed formation

Clast

Fragment of rock or mineral broken off from a large piece.

Clay

A mixture of very fine grains of micaceous substances. Clay is plastic when wet and hardens when heated. It consists mainly of hydrous aluminum silicates.

Cleavage

The splitting or tendency of a crystal to split along definite crystalline planes to produce smooth surfaces. (See Cleavage in mineral properties for more information.)

Cleavage fragment

Crystallized fragment that broke off of a mineral that exhibits cleavage.

Cleaved surface

Broken surface of a mineral that has a flat surface where the mineral broke, proving that the mineral exhibits cleavage.

Cluster

Dense agglomeration of crystals.

Coal

Black, staining, greasy, rock formed by the accumulation of herbal organic matter, such as leaves and grass.

Coating

A thin layer of one mineral on the surface of another.

Color

The characteristic color or colors of a mineral. (See Color in mineral properties)

Columnar

Aggregate defining a mineral which has parallel, slender, compact, adjoining crystals.

Composition

The elements and the quantity of the elements a substance contains.

Compound

An association of two or more elements that can only be separated by chemical means.

Concentric

Aggregate describing foliated masses that are somewhat spherical and rotate about a center; appearing like a rose (rosette). Also used to describe a form of banding where the bands are circular, forming rings about a central point.

Conchoidal

Mineral fracture in which the indentation resembles a shell. (See Fracture in mineral properties for more information.)

Concretion

Aggregate composed of a mass of small crystals that become cemented together, resulting in a rounded, odd form.

Conductivity

The ability to conduct electricity.

Conductor

1) A substance capable of transmitting electricity, such as a metal.
2) Something that is able to retain a substance such as heat or pressure.

Contact metamorphism

The process of mineral or rock metamorphism which is caused from magma intrusion near the contact with the magma.

Contact twinning

Form of twinning where two crystals join together at a base. Examples: japanese twin, spinel twin, and chrysoberyl twin.

Coprolite

Fossilized animal excrement embedded in rock.

Coral

Marine polyp that secretes calcareous skeletons. The skeleton is also called coral, and is used as an ornamental stone.

Core

The innermost layer of the earth, containing its center.

Coxcomb

Aggregate composed of flaky or tabular crystals that seem adjoined from a base; with grooves between long, slender, arc-like crystals. (i.e. Barite)

Crater

A hole that was created in the earth or celestial body from the impact of a meteor.

Crazing

Condition in Opal that causes it to form small, internal cracks, and in some severe cases will eventually disintegrate the Opal.

Crust

A disorganized, crusty, mineral coating that can be thin or thick. Type of aggregate.

Crusty

Aggregate of a crust coating on a rock or mineral.

Cryptocrystalline

Composed of tiny, microscopic crystals. See also microcrystalline.

Crystal

Any particular three dimensional form a mineral exhibits; which is classified by the distance ratio and angle of constituent parts. (See Crystals in Mineral Properties for more information.)

Crystal angle

The sum of the angles on a crystal edge that are characteristic to a crystal set. (See Crystals in Mineral Properties for more information.)

Crystal class

The method of classification used to classify the 32 different crystal types. The distinctions between the different crystal types is based on their symmetry. Crystal class in not to be confused with "crystal system" or "crystal group", which are the primary crystal classification categories. (See Crystals in Mineral Properties for more information.)

Crystal habit

The habitual form that a mineral forms its crystals as. (See Crystal Habits in mineral properties for more information.)

Crystal lattice

The arrangement of atoms in a crystal, giving each crystal its distinct shape. See also crystal structure.

Crystal structure

The framework of a crystal, usually referring to its molecular arrangement.

Crystal system

The primary method of classification of crystals. The Crystal system classifies crystals in six groups. They are: Isometric, Tetragonal, Hexagonal (which includes Trigonal), Orthorhombic, Monoclinic, and Triclinic. The crystal class, which classifies crystals into 32 crystal types, is a more precise classification of crystal groupings. (See Crystals in Mineral Properties for more information.)

Crystalline

1) Having a crystal structure.
2) Composed of visible crystals

Crystallization

The forming of crystals or to assume a crystal shape.

Crystallize

To form a crystal shape, or to have crystals in a particular group (i.e. Diamond crystallizes in the isometric system).

Cubic cleavage

Type of cleavage exhibited on minerals of the isometric system that are crystallized as cubes. The method of cleavage is that small cubes break off of an existing cube. Example = Galena.

Cyclosilicates

Group of silicate minerals that have their tetrahedrons linked into rings. Each silicon atom is bound by two oxygen atoms that are part of another tetrahedron. Each ring consists of three, four, or six linked tetrahedrons. For more information, see "All About the Silicates Group".

Cymophane effect

Phenomenon seen on a few polished gems that cause it to exhibit a floating light reflection that moves as the gem is rotated

Decomposition

The molecular breakdown of certain minerals which cause a mineral to disintegrate. (i.e. Orpiment)

Decrepitation

An explosive shattering of minerals, usually through tube tests or blowpipe tests.

Dehydration

The removal of water from a substance. Many minerals naturally lose water in their structure at normal conditions, and the mineral transforms into another mineral (i.e. Torbernite transforms to Meta-Torbernite by loss of water in its structure).

Dendrite

Mineral with a dendritic shape.

Dendritic

Aggregate composed of skeletal or tree-like formations. May be a single entity, or a formation that forms from mineral-rich solutions that deposit the mineral in rock and form a tree or plant structure embedded in rock. There sometimes is a distinction noted between the two aggregates; in some guides the former aggregate (single entity) is known as skeletal, and the latter as dendritic (embedded in rock). Other guides, such as this one, don't distinguish the two, and term them both as dendritic.

Deposit

An accumulation of certain minerals within a rock formation.

Detritus

Minerals or rock fragments that eroded and end up in a different region from natural causes, such as downstream currents.

Devitrification

Changing over from a natural glass to a mineral with a crystalline structure.

Devitrify

The process of a natural glass to lose its glassy nature and crystallize.

Diagenesis

The final stage of changing over from sediment into sedimentary rock.

Diamagnetism

Magnetic property which causes a mineral to be repelled from magnetic fields.

Diaphaneity

The quality of of a substance to be seen through. In regard to minerals, it is variable with transparency.

Diaphanous

Able to be seen through, being either transparent or translucent

Dichroic

Displaying dichroism. Minerals that exhibit dichroism are said to be dichroic.

Dichroism

Literally means "two colors". A mineral that exhibits one color when viewed from one angle but a different color when viewed from a different angle is said to display dichroism. (long green tourm)

Diffraction

The bending of light when it enters from one medium into another. For example, light bends as it travels from air into another substance, such as water. Diffraction also occurs when light enters from the air into a mineral, and the amount of diffraction varies among minerals.

Dike

A wall of igneous rock cutting across layers of surrounding rock.

Dipyramid

In form with a plane dividing a crystal into two pyramids base to base. Synonym of bipyramid. (sketch). (i.e. Zircon)

Dispersion

The splitting of white light into the colors of the spectrum. When white light enters a crystal, it splits up; some rays are reflected and some are absorbed, forming the mineral's color. Gemstones are cut in a fashion that the split-up light in the crystal fans out its colors and reflects from an exact face. This causes "fire" in a gem, which is best seen on colorless gems.

Dodecahedron

Twelve sided polyhedron; all sides are equidimensional and either rhombic or pentagonal. If the dodecahedron is composed of rhombs, it is known as a rhombic dodecahedron, or simply as a dodecahedron. If it is composed of pentagons, it is known as a pentagonal dodecahedron or pyritohedron. Minerals shaped as dodecahedrons belong to the isometric system. Plural = Dodecahdra or Dodecahedrons

Dollar

Flat, spherical disc of radiating crystals. Also a variety of Pyrite / Marcasite.

Double refraction

Phenomenon exhibited on all non-opaque minerals except for amorphous ones and ones that crystallize in the isometric system. A light ray enters the crystal and splits up into two separate rays, making anything observed through the crystal appear as double. The double refraction on most minerals is so weak that it cannot be observed without special instruments. However, in some minerals, such as the Iceland Spar variety of Calcite it is strongly seen. The double refraction is different in every mineral, and thus can be used to identify gems. Double refraction is measured with a refractometer

Doublet

One of the many gem fakes in which a thin, flat section of a real gem is pasted atop a thick base of glass or rock crystal.

Druse

Cavity in a mineral or rock filled with protruding crystals. The hole is either completely filled with crystals or just partially. (i.e. Adamite, Trap rock zeolites)

Drusy

Aggregate composed of prismatic crystals protruding from a cavity or wall.

Dry lake

Saline lake that evaporated or was drained. Dry lakes leave over many evaporite minerals, including salts, borates, and nitrates. See Dry lake deposit.

Dry lake deposit

Deposit containing an accumulation of evaporite minerals from the evaporation or drainage of a saline lake. As the water gets exhausted, the minerals it is rich in remain, increasing in content, and eventually all that is left is accumulation of the mineral that was once present in the water. See also dry lake and saline deposit.

Ductile

Capable of being stretched into a thin wire. A form of tenacity.

Ductility

The capability of being able to stretched into a thin wire; being ductile.

Dull (luster)

The luster of minerals with very poor optical properties.

Dump

Area where left over material is placed after being extracted from a mine.

Dusting

Very thin coating of one mineral on another mineral; a very thin sprinkling.

Earth science

Type of geological science dealing with the physical aspects of the earth, such as its formation, structure, and phenomenons, which include the subjects of rocks and minerals.

Earthy (fracture)

Minerals that crumble like loose sandstone when exposed to stress are said be earthy. This term may also be used to describe this crumbling property as a type of fracture.

Earthy (luster)

Luster describing minerals that are microcrystalline or amorphous and have very poor reflective surfaces.

Effervescence

A chemical reaction where bubbles of gas escape a from a liquid, which is caused by two incompatible substances. Carbonate minerals effervesce if they come into contact with hydrochloric acid.

Effervescent

Describing a mineral that effervesces in acid.

Effloresce

To lose water and develop a white powered. (See efflorescence)

Efflorescence

The phenomenon of certain minerals that when exposed to air, lose water from their chemical structure, and develop a white powder on the crystal faces. (i.e. Kernite)

Efflorescent

Describing a mineral that effloresces.

Elastic

The tenacity of certain minerals that when put under stress will bend, and will return to their original positions when the stress is released. This term is sometimes referred to as "flexible and elastic". The difference between flexible and elastic minerals is that flexible minerals will not return to their original positions after stress is released, but will develop a new position, whereas elastic minerals will return back to their original position.

Electrometallurgy

The separation of metals from ore or from alloys through an electrical process, or the forming of alloys and purification of metals through an electrical process. See also metallurgy.

Element

The classification of atoms. The different atoms are grouped as elements, which distinguishable properties are specified for each one. An element can also refers to a substance whose structure is made up of only a single type of atom. For example, the mineral Copper, which is made up of 100% the element copper (and no other substances), is known as an element. The periodical table of the elements is a layout of all the elements.

Embedded crystal

Crystal that is implanted in rock and can only be extracted if part of the rock is removed.

Empirical formula

Chemical formula that has been reduced by means of division. For example, (Si3O12) can be reduced to (SiO4) by using the lowest common divisor, which is three. Three divided by three is one {so the second formula contains one silicon (Si) molecule}, and twelve divided by three is four {so the second formula contains four oxygen (O) molecules}. See The Chemical Properties of Minerals for more information.

Enaniotropic

Describing a mineral that does not have a consistent crystal structure, meaning that it can transform back and forth into its paramorph when conditions are suitable. Monotropic minerals cannot transform back and forth into their paramorphs. Once they change, they cannot change back without the crystal structure being destroyed.

Encrustation

A crusty coating.

Encrusting

Forming a crust.

Engrave

To carve letters or an image out of a gemstone.

Environment

Area or region conducive for the development of a mineral. Certain minerals only develop in certain environments.

Epitaxy

The growth of a crystal of one mineral on or around a crystal of another mineral.

Epithermal vein

Vein formed at shallow depths from unstable hot solutions.

Epsom salts

Salt made of magnesium sulfate (MgSO4 ·7H2O) used for stomach medications. Epsom salt is extracted from the mineral Epsomite.

Erosion

Process where rock is worn away from natural procedures, such as water and wind.

Even (fracture)

Mineral fracture forming a smooth, flat surface. (See Fracture in mineral properties for more information.)

Extraterrestrial

Originating off of the earth. Meteorites are extraterrestrial.

Face

1) An individual flat section of a faceted gem.
2) Crystal face

Facet

A desired surface displayed in a gem. It may grow naturally but is usually hand cut. This definition includes the meaning of a specific cuts for gems.

Faceting

Cutting from a rough stone into a gem, creating a facet.

Fault

A split in the crust of the earth in which the two sides of the split may shift in opposing directions, resulting in an earthquake.

Feldspar

Any mineral that belongs to the feldspar group.

Feldspar group

Group of minerals that are aluminum silicates containing potassium, sodium, and/or calcium. Some minerals in this group are wrongly classified as other minerals in this group. This is the most abundant group of minerals on the earth, and the building block of many rocks.
The feldspar group is in the tectosilicates subdivision of the silicates group.
* More about the Feldspar group

Feldspathoid

Any mineral that belongs to the feldspathoid group.

Feldspathoid group

Group of minerals that are very similar to the feldspars. The difference is that the feldspathoids lacked the amount of silica to fully make them into feldspars when they were formed. They, as feldspars, are aluminum silicates of potassium, sodium, and/or calcium.
The feldspathoid group is in the tectosilicates subdivision of the silicates group.

Felsic

Consisting mostly of silica (more than 65 percent), in the form of quartz and feldspar. Can also be applied in reference only to the feldspars.

Ferromagnetic

Greatly attracted to magnetic fields.

Ferrous

Containing iron.

Ferroginous

1) Containing iron.
2) Appearing like iron.
3) Having a dull reddish-brown color similar to rust.

Fiber

Individual segment or crystal of a fibrous aggregate.

Fibrous

Aggregate describing a mineral constructed of fine, usually parallel threads. Some fibrous minerals contain cloth-like flexibility, meaning they can be bent around and feel like cotton.

Filiform

Aggregate composed of thin, intergrown, twisted, pipelike strands. (i.e. flos-ferri)

Fishtail twin

Form of repeated twinning in which two monoclinic crystals join as on the side, one reflecting the other, forming a fishtail or flower-like model (as depicted in the figure below). See also swallowtail twin

Fissile

Easily split along parallel layers.

Fissility

The property exhibited on fissile rocks or minerals that is responsible for the splitting along parallel layers. Fissility exhibited on minerals is either cleavage or parting.

Flame test

Complex, scientific test which is conducted to identify a mineral. A small fragment of a mineral is placed on the end of a platinum wire and held in a flame. Different metals present in the mineral change the color of the flame (such as sodium, yellow ; copper, blue and green ; potassium, violet).

Flaky

Aggregate containing small, flat, almost bendable flakes.

Flaw

Inclusion or crack in a gemstone that usually demotes its value.

Floating crystal

Crystal that developed without being attached to rock, such as being grown artificially in a lab or having been grown naturally in clay or water. All the sides of floating crystals are intact.

Flowstone

Calcium carbonate formed by mineral-rich water that deposits the dissolved mineral on the walls of caverns and cliffs, forming a smooth and humpy growth. As long as there is water flowing down, the layer of flowstone accumulates. See also Calcite, Aragonite, Stalactites and Stalagmites

Flow banding

Layered grouping of different rocks formed from the flowing of lava.

Fluorescence

Property of certain minerals in which it displays a multicolored effect when having ample illumination with ultraviolet light.(See Fluorescence in mineral properties for more information)

Fluorescent lamp

Lamp that gives off ultraviolet light while blocking out white light, which causes fluorescent minerals to react.

Flux

Any liquid added to another liquid to improve flow, usually to prevent the formation of oxides.

Foliated

Aggregate compose of numerous, thin, leaf-like crystals. (i.e. Hematite)

Form

The setting of all the crystal faces and the structure of a mineral. See also crystal form.

Fossil

Animal or plant remains of a previous age embedded and preserved in rock.

Fourling

Twinned intergrowth of four individual crystals. See twin.

Fracture

The characteristic way a mineral breaks when put under stress, aside from cleavage.

Friable

A mineral which easily crumbles is friable. Minerals with an earthy fracture are friable.

Fumarole

Volcanic vent where smoke and gases escape from.

Fusion

The process of being melted by heat.

Gamma rays

Form of electromagnetic radiation which contains the shortest wavelength of the spectrum. Gamma rays easily penetrate matter because of their short wavelength. See also alpha and beta waves.

Gangue material

Worthless material in which valuable metals or gems occur.

Garnet paper

Sandpaper composed of tiny, hard, Garnet crystals, glued on to it for use as an abrasive.

Geiger Counter

Instrument that measures intensity of radiation. It is very useful for detecting radioactive minerals.

Gem

A cut mineral or pearl refined for use as an ornament.

Gemology

The science and procedures involved in mining, faceting, and marketing of gemstones.

Gemologist

Individual practicing in the subject of gemology.

Gemstone

Any mineral or naturally occurring substance in an raw, uncut state that is capable of being a gem.

Geode

Hollow rock that is filled or partially filled with crystals.

Geologic

Having to do with geology.

Geology

The science and study of the physical structure of the earth, including the areas of rocks and minerals.

Gneiss

Banded or layered metamorphic rock, same in composition to Granite.

Glauber's salt

Salt made of sodium sulfate (Na2 SO4 ·10H2O) used in the manufacture of paper ,glass and in stomach medications. Glauber's salt is extracted from the mineral Glauberite.

Globular

The term globular is used as a synonym of botryoidal, but sometimes describes any rounded agglomeration, such as botryoidal, reniform and mammilary.

Goniometer

Instrument that measures crystal angles.

Grain

Individual component belonging to a grainy aggregate.

Grain

Weight used in reference to measuring pearls. A grain is .05 grams or ¼ carat.

Grainy

Crystal aggregate resembling a cluster of grain.

Gram

Weight measurement used to measure less valuable gems or rough stones. It corresponds to the measurement of the metric gram.

Gravel

Accumulation of small, smooth, rounded rock deposited by a stream or river.

Gravity

The force that attracts matter to the center of the earth. The term gravity is sometimes used as a shortened form of the term specific gravity.

Greasy (luster)

Luster of a mineral that appears coated with grease. Some minerals are coated with chemicals to induce a greasy luster.

Group

1) An aggregate of crystals.
2) Synonym of crystal group.
3) The classification order that minerals are arranged in based on their chemical structure.
4) A scientifically recognized selection of minerals similar in structure. All groups have a specific group name, and individual minerals may also be categorized. A mineral in the group may have the same name as group name. An example is the Olivine Group, which contains the minerals Forsterite, Olivine (also known as Chrysolite), and Fayalite. See also series.

Habit

The attributes of the appearance of a crystal or aggregate.

Hackly (fracture)

Type of fracture resembling broken metal, exhibiting sharp, jagged surfaces. This fracture is sometimes known as "jagged" fracture

Halides

Group of minerals containing one of the halogen elements (chlorine, fluorine, bromine, and iodine) as a building block. Most halides are soft and fragile, and some are soluble in water. Many crystallize in the isometric system.

Halogen

Five chemically related elements belong in the halogen group. They are astatine, bromine, chlorine, fluorine, and iodine. Minerals that are composed of the halogen elements are known as halides. (Astatine is a synthetic element and there are no minerals that contain it in their structure.)

Hardness

The resistance of an object to scrapes and scratching. The harder it is, the greater its resistance. (See Hardness in mineral properties for more information.)

Hardness testing kit

Kit composed of minerals or rods with labeled harnesses used to scratch a mineral to test its hardness.

Hard water

Water rich in calcium.

HCl

Abbreviation and chemical formula of hydrochloric acid.

Heat treated

Said of a mineral or gem put under intense heat to enhance color or remove flaws.

Hemimorphic

Said of a doubly terminated crystal with two differently shaped ends.

Hexagon

Polyhedron with six sides and a top and bottom base.

Hopper

Crystal form exhibiting an indenting, terraced, structure penetrating towards the center.

Host mineral

Mineral that is the chief constituent of a particular rock.

Hydrate

To absorb water and construct it as part of the crystal lattice.

Hydration

The addition of water into a minerals' chemical structure.

Hydrocarbons

Organic substances that are composed only of hydrogen and carbon.

Hydrochloric acid

(HCl) Corrosive acid used mainly for dissolving unwanted substances. It is a very destructive liquid and will destroy many minerals. See also nitric acid and acid test.

Hydrofluoric acid

(HF) Corrosive acid used for the production of glass(, since it dissolves glass). It is a very destructive liquid and will some many minerals.

Hydrometallurgy

The separation of metals from ore or from alloys through a process in which a liquid is the primary factor, or the forming of alloys and purification of metals through a process in which a liquid is the primary factor. See also metallurgy.

Hydrothermal

"Hydro" means Water and "Thermal" means Heat. Hydrothermal solutions are solutions of hot water arising from underground sources. They may be as cool as cold water or hot as burning hot steam.

Hydrous

Containing water, and in some cases, containing hydroxyl.

Hydroxides (subgroup)

Compounds of metallic elements combined with water (H2O) or hydroxyl (OH). The hydroxides are a subgroup of the oxide group.

Hydroxyl

Radical composed of hydrogen and oxygen. formula = (OH)

Hygroscopic

Describing a mineral that intakes and retains water from the atmosphere, and forms part of its structure. Hygroscopic minerals should be kept away from humid areas and water, and should preferably be kept in rice or silica gel which absorb moisture, since water can destroy such minerals.

Hypothermal vein

Vein created at extreme depths and at a very high temperature.

Icosahedron

Twenty sided polyhedron; all sides are equidimensional. Minerals shaped as icosahedrons belong to the isometric system. Plural = Icosahedra or Icosahedrons

Igneous

Of volcanic origin

Igneous rock

A type of rock from volcanic origins. Igneous rock can be glassy, crystalline, or both.

Impure

Containing impurities.

Impurity

An item present in a mineral which is not part of its integral structure, and may change its optical properties, such as color.

Inclusion

Materials that are locked inside a mineral as it is forming. (i.e. Rutilated Quartz)

Incrust

To form a crust over.

Incrustation

A pseudomorph occurring when a mineral forms a coating over another mineral, and the coated mineral dissolves. This leaves a hollow cast of the mineral that coated the dissolved mineral.

Infiltration

A pseudomorph occurring when some atoms of a mineral are replaced by different atoms forming another mineral.

Inosilicates

Group of silicate minerals that have their tetrahedrons form single or multiple chains, with two oxygen atoms of each tetrahedron part of its neighboring tetrahedron forming long, thin, chains. For more information, see "All About the Silicates Group".Group of silicate minerals that have their tetrahedrons form single or multiple chains, with two oxygen atoms of each tetrahedron part of its neighboring tetrahedron forming long, thin, chains. For more information, see "All About the Silicates Group".

Inorganic

Not containing any organic substances.

Intaglio

Gemstone carved into a cameo, but instead of the engraving raised above the background, as in the cameo, the engraving is etched into the background. Intaglios were used as royal seals.

Intermediary

Describing a mineral midway between two mineral of a series. For example, the mineral Olivine - (Mg,Fe)2SiO4 - is midway between Forsterite - Mg2SiO4 - and Fayalite - Fe2SiO4 - in the Olivine Group.

Intrusive rockIntrusive roc

Igneous rock that was formed underground, from magma that was stuffed into cracks.

Ion

Atom that becomes positively or negatively charged due to the loss or gain of an electron. (See anion, cation)

Iridesce

The present term describing something displaying iridescence.

Iridescence

Light effect causing a mineral to display a play of colors on an apparently monocolored surface. Iridescence is many times the result of pearly luster, seen around an area where pressure occurred, displaying a similar image to that of fresh oil rising to the surface of a road at the beginning of a rain. Iridescence is also the result of mild tarnishing of a few metallic lustered minerals, such as Chalcopyrite and Hematite.

Irradiation

The act of being exposed to radiation. This may have an effect on several gemstones by altering their color.

Isometric (crystal system)

Any mineral that falls under the following specifications belongs to the isometric crystal system: Three axes, all of them are equal in length and lie at 90° from the other. (See Crystals in mineral properties for more information). The term isometric is also used to describe any mineral equal in all three dimensions.

Isomorph

Describing a mineral as being isomorphous to another.

Isomorphous

Having the same crystal form, meaning that the molecular arrangement is identical, except for the fact that different elements are present. If two minerals are isomorphous to each other, than they contain different elements arranged in the same arrangement and number, such as Calcite (CaCO3) and Siderite (FeCO3).

Isomorphism

The concept of minerals being isomorphous to one another.

Isotropization

Radioactive decaying that causes destruction to a minerals crystal lattice.

Ivory

Ornamental material made out of the calcareous teeth of many mammals, such as elephants, sea lion, hippopotamus, narwhal, and boar.

Japanese twin

Form of contact twinning where two single Quartz crystals are joined by the their base at an angle near 90°.

Jet

Coal that can be polished and used for ornamental purposes.

Jewel

A gem, whether cut or uncut, capable of being used as an ornament.

Jeweler

Individual whose trade is to buy and sell jewels and gemstones.

Jewelry

Jewels worn as ornaments.

K. or Kt

Abbreviation of karat.

Karat

Unit of measurement describing the content of gold in an ornament. The karat unit measures the percentage of gold in metallic objects made of gold (mainly jewelry). Since pure gold bends too easily, it is mixed with tougher metals when made into ornaments to form a tougher, although impurer, gold. The karat measurement determines the percentage of gold on a 1 to 24 scale, with 24 karats being pure gold. That means that 14 karat gold is only 14/24 (or 58.33%) gold. Gold with a high karat measurement is undesirable for jewelry because it is soft and bendable, and gold with a low karat measurement is undesirable because of its low gold content [hence lesser value]. The term karat can be confused with the term carat, which is the weight measurement for gemstones. The abbreviation of karat is k. or kt.

Laboratory

Room or complex containing scientific equipment used for research and tests.

Labradoresce

The present term describing something displaying labradorescence.

Labradorescence

Effect which causes dark, metallic-like color shimmers, commonly blue and green, to be displayed on a few minerals. The name is derived from Labradorite, a mineral which is the best example of this effect.

Laccolith

Lenticular mass of igneous rock stuck between layers of sedimentary rock.

Laminated rock

Sedimentary rocks consisting of many thin layers.

Lanthanides

See rare earth elements

Lapidary

An individual who facets gemstones as a trade, and the shop of such an individual. Also used in adjective form when relating to gems (i.e. lapidary equipment).

Lava

Molten rock on the surface of the earth released from a volcano.

Lenticular

Lens shaped. When applied to minerals it refers to concretions or nodules that have a flattened, lens-like shape

Litmus paper

Paper with powder extracted from certain plants that tests the pH of a substance. Acid turns red while base turns blue; neutral remains white. The stronger the acid or base, the more intense red or blue the color of the litmus paper turns.

Locality

Area where a specific mineral was found or occurs.

Lode

Vein of precious metal.

Longwave

A wavelength of ultraviolet light. Some minerals display fluorescence when exposed to longwave ultraviolet light. (See Fluorescence in mineral properties for more information). See also shortwave. Commonly abbreviated as LW.

Loupe

Small magnifying lens used to observe certain aspects of a gemstone. Loupes usually have a magnification level of 10x.

Lubricant

Material that can reduce friction. Used as a coating on objects that are subject to friction.

Luminesce

To give off light under certain conditions. See luminescence.

Luminescence

General term describing the capability of a mineral with a habit of giving off light when put under certain conditions. Three examples of luminescence are fluorescence, triboluminescence, and thermoluminescence.

Luminescent

General term describing any mineral capable of giving off light when put under certain conditions. Three examples of minerals that are luminescent are minerals that exhibit fluorescence, triboluminescence, and thermoluminescence.

Luster

The exhibition of the surface of minerals, including their reflective properties. (See luster in mineral properties for more information.)

Macrocrystalline

Having crystals large enough to be seen with an unaided eye.

Mafic

Describing dark colored rocks or minerals that are composed particularly of magnesium and iron.

Magma

Molten rock under the crust of the earth.

Magmatic

Igneous.

Magmatite

Term describing any igneous rock.

Magnesia

1) Industrial material made of magnesium oxide.
2) Describing a mineral that is composed of magnesium oxide.

Magnet

An object that is surrounded by a magnetic field, causing iron or steel materials to be attracted to it. (See magnetism for more information.)

Magnetic

This term can be used to describe materials that give off magnetic fields (i.e. act as a magnet), or materials that are attracted to magnetic fields. To note the distinction, this guide defines "magnetic" as giving off a magnetic field, and "attracted to magnets" as being attracted to a magnetic field.

Magnetic field

Area encompassing a magnet or electric current which has the ability to attract or repel certain objects anywhere in the field. Points in the magnetic field closer to the magnet or electrical current are stronger, and points further away are weaker.

Magnetism

The phenomenon of certain metals to attract other metals. (See Magnetic for more information.)

Malleable

Tenacity of a certain minerals that are able to be pounded into thin sheets.

Mammilary

Aggregate describing smooth, rounded, agglomerations. Rounded agglomerations of mammilary aggregates are larger than reniform agglomerations and considerably larger than botryoidal agglomerations.

Mantle

The middle layer of the earth -- between the crust and the core.

Massive

Term used to describe a rock or mineral that has no particular shape, either because

1. it is non-crystalline
2. it is composed of tiny, unorganized crystals
3. it is a shapeless fragment of a crystal

Matrix

A material that has an embedded crystal inside or emerging from it.

Melting point

The temperature it takes a certain material to change from a solid to liquid state.

Mesothermal vein

Vein created due to intense heat (in range from around 390° to 570° F [200° to 300° C] ).

Metal

Any of a category of electropositive (positively charged) elements or combinations of them in the form of minerals that exhibit a metallic luster, malleability, ductility, and conductivity.

Metallic

Having the attributes of a metal. Sometimes can be used to describe a mineral with a metallic luster even though it is not a true metal.

Metallic (luster)

Exhibiting the luster of a metal, which is opaque and reflective. Some minerals exhibit a metallic luster even though they are not true metals.

Metallic elements (subgroup)

The metallic elements are minerals belonging to the native elements group. They consist of minerals that are pure elements or metallic alloys. They are all true metals, that is they have a metallic luster, are malleable and ductile, and are good conductors of electricity.

Metallurgy

The science and procedures involved in extracting metals from ore, refining the metals, blending them into alloys, and fashioning useful objects from them. Three forms of metallurgy are hydrometallurgy, electrometallurgy, and pyrometallurgy.

Metamict

Crystalline minerals that lose their crystal structure due to radioactive destruction.

Metamiction

The process which a metamict mineral goes through.

Metamorphic

Mineral environment where the minerals are secondary in origin, forming from alteration through heat and pressure. See also metamorphic rock.

Metamorphic rock

Secondary rock that formed from an original rock, through heat or pressure.

Metasilicates

This term either refers to the metasilicate chains (inosilicates), the metasilicate rings (cyclosilicates), are both the metasilicate chains and rings.

Metasomatism

The chemical alteration of rocks or minerals by interaction with liquids. The alteration must only take place if the rock or mineral was not in a molten state in order for it to be metasomatism.

Metastability

The act of being metastable.

Metastable

Describing the condition of radioactive minerals which cannot withstand their composition and begin to decay. Also refers to minerals that crystallize at a certain temperature and can only retain the initial crystal structure at certain temperatures. Synonym of unstable

Meteor

Mass of extraterrestrial rock that gets pulled in to the earth's gravitational pull. Some meteors can be seen streaking across the atmosphere before they burn up or land on earth. These are known as "shooting stars". Meteors that don't burn up and land on the earth are known as meteorites.

Meteoroid

Solid mass suspended in outer space that revolves around the sun. Meteoroids are smaller than asteroids. See also meteor.

Meteoric

Term used to describe the extraterrestrial nature of meteors and meteorites.

Meteorite

Meteor that did not fully burn up in the atmosphere and landed on the earth. (See Iron/Iron-Nickel for more information)

Mica

Individual member of the mica group. May also refer to any mineral with perfect basal cleavage that can be "peeled".

Mica group

Group of related minerals that contain aluminum silicate. The percentage of its component elements may vary, which causes many people not to recognize the micas as a series. The micas are soft and have perfect basal cleavage, and can be seen when a mica is "peeled". The mica group is in the phyllosilicates subdivision of the silicates group.

Micaceous

Aggregate of compact, flat, parallel, flexible, and peelable sheets, or describing minerals that occur in such aggregates.

Microcrystalline

Composed of tiny crystals that cannot be seen with an unaided eye. Microcrystalline minerals appear amorphous, since no apparent crystal shape can be detected.

Micromount

A mineral specimen that is not more 1/10 of an inch (15 -27 mm.) in size. (See organizing a collection for more information about micromounts.)

Mine

Deposit in which minerals or ore is or was industrially extracted. (See Finding minerals for more information about mines.)

Miner

Individual who exploits mineral deposits.

Mineral

Any naturally occurring, three dimensional, inorganic substance, with a chemical structure that can be exact, or can vary within limits. Elements that occur naturally are also listed as minerals. See What is a Rock and What is a Mineral for more information.

Mineral group

A scientifically recognized selection of minerals similar in structure. All groups have a specific group name, and individual minerals may also be categorized. A mineral in the group may have the same name as group name. An example is the Olivine Group, which contains the minerals Forsterite, Olivine (also known as Chrysolite), and Fayalite. See also series.

Mineraloid

Amorphous, inorganic substance lacking a crystal structure. They are mineral in nature but lack a crystal structure, and thus cannot scientifically be classified as minerals. Many references, though, still classify them as minerals. Two important mineraloids are Opal and Obsidian.

Mineralogy

The study of minerals.

Mineralogist

Individual practicing in the subject of mineralogy.

Mining

The process of extracting minerals or metal ore out of a mine or mineral deposit.

Mixed crystal

Crystal containing an indefinite amount of two or more elements or a slight amount of one element replacing another. An example is Siderite, which is iron carbonate (FeCO3), but commonly contains small amounts of other elements, such as zinc and manganese partialy replacing the iron. See also solid solution.

Mohs Hardness scale

A measurement that was devised by Austrian scientist Fredrick Mohs to determine the hardness of a mineral. (See Hardness in mineral properties for more information.)

Molecule

The fundamental structure in all minerals. Molecules are chemically grouped atoms that are the smallest particles a mineral can be divided without changing its chemical or physical properties. A chemical grouping of one element is also considered a molecule.

Molecular arrangement

The arrangement of molecules in a substance.

Molecular structure

The quantity and method of arrangement concerning the molecules in a particular substance.

Mollusk

Family of marine creatures, which includes the oyster and snail, which have a soft fleshy body surrounded by a calcareous shell.

Molten

Hot liquid that results from the melting of solid material at great temperatures.

Molten rock

Liquid rock at extremely high temperatures under the surface of the earth. When molten rock cools down it solidifies and forms rocks and minerals. (See magma.)

Molybdates

Group of minerals composed with the molybdate radical (MoO4) and a metallic element. These minerals are heavy, soft, and brittle. The molybdates are usually categorized with the chemically related tungstates, in which they can be partially replaced by.

Monochromatic

Exhibiting only a single color. Minerals that are monochromatic occur in only one color, no matter what specimen.

Monoclinic (crystal system)

Any mineral that falls under the following specifications belongs to the monoclinic crystal system: Three axes, all of them are unequal in length. Two of them are at right angles to each other, while the third is lies at an angle other than 90°. (See Crystals in mineral properties for more information) Animated figure of monoclinic crystal system

Monotropic

Mineral that permanently changes over into its paramorph, meaning that once it transforms under suitable conditions it cannot change back to the original mineral without the crystal structure being destroyed. Enaniotropic minerals can change back and forth when conditions are suitable.

Mother-of-pearl

The iridescent inside of a mollusk shell, which is used as an ornament. Not to be confused with pearl.

Mother rock

The rock a mineral or minerals is found implanted in.

Mottled

Spotted or speckled with different tints or colors.

Multicolored

Exhibiting two or more colors on a single specimen.

Multiple oxides (subgroup)

Minerals that are compounds of two different metallic elements combined with oxygen. The multiple oxides are a subgroup of the oxides.

Mylonite

Rock fragments separated from their original rock from shifting of plates in the fault zones.

Native

Composed of only a single element; not combined with any other elements.

Native elements (group)

Group of minerals containing naturally occurring minerals with a molecular structure of only one element; examples are Copper, Sulfur, and Diamond. Also included in this group are metallic and semi-metallic alloys, which are minerals composed only of two or more metallic or semi-metallic elements of varying percentage; two examples are Iron-Nickel (Fe,Ni) and Allemontite (As,Sb). The minerals Gold and Platinum are considered native elements even though they are never 100% pure gold or platinum, containing indefinite traces of other elements, thereby causing them to be metallic alloys. The native elements may be divided into three groups: The metallic elements, The semi-metallic elements, and the non metallic elements.

Natural Glass

Igneous rocks that forms when rapid cooling of molten rock occurs. Natural glasses, such as Obsidian, are amorphous with a rounded shape, and usually contain conchoidal fractures.

Nesosilicates

Group of silicate minerals that contain only single, non-combining groups of tetrahedrons. For more information, see "All About the Silicates Group".

Neutron

Neutrally charged particle of an atom that resides in its nucleus.

Nitrates

Group of minerals that contain one or more metallic elements plus the nitrate radical (NO3). These minerals are all fragile and soft. With one exception, they are all soluble in water, and are therefore found only in arid regions, primarily in dry lake deposits. The nitrates are a small group, and are sometimes classified as a sub-category of the carbonate group.

Nitric acid

(HNO3) Corrosive acid used in the manufacture of explosives and fertilizers. It is a very destructive liquid and will destroy many minerals. See also hydrochloric acid and acid test.

Nodular

Spherical, in the shape of a small rounded lump.

Nodule

Aggregate consisting of a spherical lump, usually from groups of small crystals.

Nonbrittle

Not brittle, meaning not crumbling to a powder when crushed.

Nonmetal

Substance that does not exhibit the properties of a true metal.

Non-crystalline

Not containing any crystals (i.e. amorphous mineral or variety of mineral).

Non-metallic elements (subgroup)

The nonmetallic elements are minerals that belong in the native elements group and don't exhibit any metallic properties (except for Graphite, which has a metallic luster). They are light in weight and can be transparent.

Nuclei

Plural form of nucleus.

Nucleus

The center of an atom that contains the protons and neutrons.

Nugget

Compact, waterworn, amorphous mass, found in placer deposits.

Occurrence

The area where a particular mineral is found.

Ochre

Amorphous, yellow to red substance composed of iron compounds such as Hematite and Limonite.

Octacube

Three dimensional polyhedron that is a combination of a cube and octahedron.

Octahedral

Shaped as an octahedron.

Octahedral cleavage

Type of cleavage exhibited on minerals of the isometric system that crystallize as octahedrons. The method of cleavage is that flat, triangular "wedges" peel off of the existing octahedron. Example = Fluorite.

Octahedron

Eight sided polyhedron; all sides are equidimensional and bisect at at the same angle. Minerals shaped as octahedrons belong to the isometric system. Plural = Octahedra or Octahedrals

Oolite

Any mineral that is Oolitic.

Oolitic

Aggregate composed of very small, spherical particles.

Opalesce

The present term describing something displaying opalescence.

Opalescence

Effect seen in a few minerals, chiefly opal (hence its name) which cause it to exhibit a glimmer of different colors when rotated or seen in different angles.

Opalization

The common habit of Opal that it replaces material, such as wood, shells, and other minerals.

Opalize

To be chemically altered to Opal. See Opalization

Opaque

Not able to transmit light, which in effect disables it from letting an object be seen through it. (See Transparency in mineral properties for more information.)

Optical

Having to do with optics.

Optical properties

Physical properties of a mineral or gem that have to do with optics, such as dispersion, absorption spectra, refractive index, asterism, and dichroism, just to name a few.

Optics

Branch of physics that deals with light and the electromagnetic radiation; certain areas are dispersion, absorption spectra, reflection, and refraction.

Ore

Material which has a valuable constituent, usually a precious metal, that gives it value and makes it profitable for extraction. A fine example is Calaverite, which is extracted because of the gold it contains. "Ore" also refers to the precious material that was extracted from the ore.

Organic

Composed of carbon compounds; being from the source of living organisms.

Orthorhombic (crystal system)

Any mineral that falls under the following specifications belongs to the orthorhombic crystal system: Three axes, all are unequal in length. All three axes are at 90° to each other. (See Crystals in mineral properties for more information)

Outcrop

Bedrock revealed at the surface of the earth.

Oxidation

The process of undergoing a chemical change through exposure to oxygen. See also oxidize.

Oxidation minerals

Minerals that form after being altered from being exposed in the oxidation zone.

Oxidation zone

Area of a deposit where the rock is exposed to air and therefore is affected by wind, rain, pressure, and air, which chemically affects the minerals embedded in the rock and alters them to secondary minerals.

Oxides

Group of minerals that are compounds of one or more metallic elements combined with oxygen, water, or hydroxyl (OH). The oxide group contains the greatest variations of physical properties. Some are hard, some soft. Some have a metallic luster, others are clear and transparent. The Oxide group is divided into the Simple Oxides Hydroxides Multiple Oxides

Oxidize

Property exhibited in certain minerals that cause them to tarnish, or discolor upon contact with air. Also term used to describe the chemical alteration of one mineral into another mineral through oxidation.

Oxidized

Having gone through a chemical change as a result of exposure to oxygen.

Paleontology

The science and study of previous life forms, primarily in the form of fossils.

Paleontologist

Individual practicing in the subject of paleontology.

Paragenesis

The crystallization mode of one mineral in retrospect to other minerals. Paragenesis is applied to determine the conditions and qualifying factors necessary for a mineral to form.

Paramagnetic

Weakly attracted to magnetic fields.

Paramagnetism

Magnetic property in certain iron bearing minerals that cause them to be weakly attracted to magnetic fields.

Paramorph

A pseudomorph involving two minerals with an identical composition but different crystal structures. The original mineral forms, but conditions then cause it to be unstable, so it transforms into the other mineral with the same chemical structure while retaining the original crystal shape. An example of this is Aragonite (CaCO3 - Crystallizes in orthorhombic system) that becomes unstable and transforms into Calcite (CaCO3 - Crystallizes in trigonal system), but retains the original orthorhombic shape.

Parting

The tendency of certain minerals to split along stressed areas or along twinned crystals. (See Parting in mineral properties for more information.)

Pearl

Smooth, round, shiny, organic object composed mainly of calcium carbonate found in the shells of some mollusks. Not to be confused with mother-of-pearl.

Pearly (luster)

Exhibiting a luster similar to the inside of a mollusk shell or shirt button. Many mica's exhibit a pearly luster, and some minerals with a pearly luster have an iridescent hue. Some minerals may exhibit a pearly luster on cleavage cracks parallel and below the reflecting surface of a mineral.

Pegmatite

Area of igneous rock where the individual grains are very large, meaning that large crystals occur there.

Pendant

Ornament or piece of jewelry that hangs down, such as from a necklace or earrings.

Penetration twinning

Form of twinning where two or more crystals are intergrown. Examples: fluorite twin, carlsbad twin, staurolite twin, and swallowtail twin.

Penetration

The formation of a crystal penetrating through rock or another crystal.

Percussion figure

Six rayed, star-like flaw that forms when some micaceous minerals are put under pressure.

Petrification

Process in which organic substances, such as wood and shells, are replaced by silica.

Petrified

Having been replaced by silica after once being organic. See also petrification. Petrified Wood

Petrified wood

Wood that is petrified (i.e. replaced by silica).

Petrographer

Individual practicing in the subject of petrography.

Petrography

Type of geology that deals with the classification of rocks, which is based on the material they contain.

Petrographic microscope

Special microscope used to determine the makeup of rocks in order to classify them. See petrography.

Petrology

Type of geology that deals with the formation, composition, and source of rocks.

Petrologist

Individual practicing in the subject of petrology.

pH

Measurement used to survey the amount of acid or alkaline present in a material. pH is measured on a scale of 0 - 14. Neutral is 7; numbers below 7 are acid and above are alkaline. The lower the number, the stronger its acidity, and the greater the number, the stronger the alkalinity. pH stands for "potential of Hydrogen" (for hydrogen causes a material to be acidic).

Phantom growth

An interesting phenomenon exhibited when a crystal grows, than a new growth grows over the old crystal in the same direction, leaving an inscription of the previous growth on the crystal. Additional growth may be present, leading to the possibility of more than one phantom in a crystal.

Phenocryst

Large crystal surrounded by much smaller crystals in porphyritic igneous rock.

Phosphates

Group of minerals that are compounds of one or more metallic elements associated with the phosphate radical (PO4). The phosphates are classified together with the arsenates and vanadates. Most phosphates are heavy, and none are hard. They are usually brittle and occur in small crystals or compact aggregates. (In this guide, "true phosphates" refers only to minerals with the phosphate radical, excluding the arsenates and vanadates which contain the arsenate and vanadate radicals.)

Phosphorescence

The ability of some fluorescent minerals to keep on glowing for several seconds after the ultraviolet source has been removed. (The phosphoresce may cause the mineral to glow for only several seconds, but the phosphorescent effect really lasts much longer -- several seconds is only what the eye can perceive.

Phyllosilicates

Group of silicate minerals that have each set of tetrahedrons surrounded by three oxygen atoms, forming a sheet like structure. For more information, see "All About the Silicates Group".

Physicist

Individual who specializes in physics.

Piezoelectric

Substance that generates an electrical charge when under stress.

Piezoelectricity

Electricity generated when a piezoelectric substance is put under stress.

Pinicoid

The third dimension of a three dimensional figure, usually representing its width.

Pinicoidal

By way of the pinicoid. When in reference to a crystal type, it refers to an elongated crystal.

Pinicoidal cleavage

Type of cleavage exhibited on some prismatic and tabular minerals where they cleave on the pinicoidal plane, which is the third dimension aside from the basal and prismatic sides, which they may also cleave on.

Pisolite

Any mineral that is pisolitic.

Pisolitic

Aggregate composed of small, spherical particles, larger in size and commonly more distorted than oolitic minerals.

Pit

Type of mine where a large hole is dug in the ground to extract the valuable material.

Pitchy

Luster of a mineral that appears similar to tar. Minerals with a pitchy luster are radioactive and have gone through the process of metamiction.

Placer deposit

Area in a stream or river where heavy material settles after being carried downstream in the current of the river or stream.

Plagioclase twin

Two or more crystals that twin in a repeated pattern ("repeated twinning") as depicted in the figure below. Named after the mineral Plagioclase, which most frequently exhibits this form of twinning.

Plane

Imaginary line connecting two points on a surface.

Plane of symmetry

Imaginary lines traced on polyhedrons such as cubes and octahedrons depicting a point on the polyhedron that exhibits symmetry (i.e. if rotated 180° from that line will yield the same shape).

Plate

Small, flat, flaky crystal. Also the definition of the section of rock present by fault areas.

Platy

Small, flat, and flaky.

Playa

A flat plain in a low point in a desert where rain collects after a storm.

Pleochroic

Displaying pleochroism.

Pleochroism

The effect present in a mineral exhibiting two or more separate colors when viewed at different angles. Pleochroism and dichroism are synonymous, except dichroism refers only to two colors, but pleochroism can be more than two.

Plug

The solidified "cover" over the opening of a dormant volcano.

Plumose

Featherlike aggregate in which many small crystals protrude out of a long, slender one.

Plutonic rock

Igneous rock that solidified in the crust of the earth, and individual crystal grains can be seen.

Pocket

Cavity in igneous rock in which crystals are usually found.

Point

Unit of measurement given to small, precious gemstones. One point is equivalent to 1/100th of a carat. The abbreviation for point is Pt.

Polished

Either tumbled, faceted, or coated to enhance luster.

Polychromatic

Minerals that are polychromatic have many different color variances.

Polyhedron

A three dimensional figure composed of specific shapes.

Polymorph

A mineral with the chemical equivalence (i.e. containing the same amount of the same elements) of another mineral but different atomical arrangement. Two examples are:
1) Diamond and Graphite
2) Rutile, Brookite, and Anatase

Polymorphism

The tendency of minerals with the same chemical composition to form different crystal structures.

Polymorphous

Being polymorphs to one another.

Polysynthetic twinning

Form of twinning where the crystals intergrow in a repeated pattern.

Polysynthetic twins

Twinned crystals that are twinned through polysynthetic twinning.

Porcelaneous

Containing rounded, tiny holes throughout.

Porous

Containing rounded, tiny holes throughout. Many porous minerals can be dyed.

Porphyritic

Describing a rock that contains large, noticeable crystals, usually feldspars.

Porphry

Igneous rock containing large, noticeable crystals, usually feldspars.

Precious stone

Gem or gemstone that is highly appealing and very costly, exhibiting a powerful luster, high hardness, and rarity.

Precipitating

Going through precipitation, meaning releasing dissolved mineral.

Precipitation

The process in which dissolved mineral gets freed from water, forming a deposit..

Primary

Forming as new, not as an alteration product. (See "primary mineral" for more information)

Primary mineral

Mineral that forms by the combination of elements rather than by alteration of a mineral.

Prism

Crystal that is elongated in one direction; the other directions are about equal.

Prismatic

Crystal habit describing a crystal with four or more sides similar in length and width. Prismatic crystals are usually elongated in one direction.

Prismatic cleavage

Type of cleavage exhibited on some prismatic minerals in which the mineral cleaves by breaking off thin, vertical, prismatic crystals off of the original prism. Example = Acmite.

Prospect

To search for a mineral deposit or mineral in a deposit.

Prospecting

Searching for a mineral deposit or a mineral in a deposit.

Prospector

Individual who prospects minerals.

Proton

Positively charged particle of an atom that resides in its nucleus.

Pseudo

Assuming a false shape.

Pseudocube

Rhombohedron shaped crystal almost identical to a cube, but its angles slightly differ from a cube. (May also refer to any crystal that closely resembles any member of the isometric system but is slightly asymmetrical.)

Pseudocubic

Assuming the shape of a pseudocube.

Pseudohexagon

Six sided crystal that assumes a hexagonal shape although it is not in the hexagonal system. The cause of pseudohexagonal crystals is orthorhombic crystals that intergrow in three individuals, forming six sided trillings.

Pseudomorph

One mineral that chemically replaces another mineral without changing the external form of the original mineral. There are three types of pseudomorphs: paramorphs, infiltration pseudomorphs, and incrustation pseudomorphs.

Pseudomorphed

A mineral that has gone through pseudomorphism.

Pseudomorphism

The act of one mineral chemically replacing another. See also pseudomorph.

Pure

Free of impurities.

Pyncnometer

Type of instrument that measures specific gravity.

Pyramidal

Shaped as a pyramid (i.e. with a pointed top).

Pyritohedral

Shaped as a pyritohedron

Pyritohedron

Twelve sided polyhedron; all sides are equidimensional and pentagonal. Minerals shaped as pyritohedrons belong to the isometric system. Plural = Pyritohedra or Pyritohedrons

Pyroelectric

Substance that generates an electrical charge during a temperature change.

Pyroelectricity

Electricity generated when a pyroelectric substance is put under temperatures in which it will generate electricity.

Pyrometallurgy

The separation of metals from ore or from alloys through a process dependent on heat, or the forming of alloys and purification of metals through a process dependent on heat. See also metallurgy.

Pyroxene

Individual member of the pyroxene group.

Pyroxene group

Group of minerals that contain iron, magnesium, silicon, and oxygen. They may also contain calcium, sodium, and lithium. The pyroxenes have prismatic cleavage, and one can note the angle of 87° and 93° (almost perpendicular), which distinguishes it from amphiboles which are at 56° and 124°. The pyroxenes are very similar to the amphiboles, and the minerals belonging to each group many times cannot be distinguished by ordinary methods. Generally, though, the pyroxenes occur in shorter, stubbier crystals than the amphiboles. The amphiboles contain the same elements that the pyroxenes do, except they have hydroxyl (OH) in their structure, which alters both its physical and chemical properties. The pyroxene group is in the inosilicate subdivision of the silicates group.

Quarry

Mine in which minerals or ore is blasted out of the side of a mountain or hill.

Radiating

Aggregate composed of tiny, slender crystals compacted together radiating from a central point. The radiation can be flat or three dimensional. If three dimensional, this aggregate commonly occurs with circular, ball-like masses, and is known as spherulitic.

Radiation

The emission of alpha, beta, or gamma rays.

Radical

Compounds that act as a single atom when combining with other elements to form minerals. Radicals contain one or more unpaired electrons.

Radioactive

1) Term given to describe substances that contain atoms whose nuclei are are unstable, which causes the substance to slowly decay and emit radiation. Radioactive minerals usually take an extremely long time to decay if kept in proper settings.
2) (verb) - Emitting alpha, beta, and gamma rays.

Radioactivity

The effect of radioactive material, emitting alpha, beta, and gamma rays. Also used to describe in present tense the nature of radioactive material.

Rare earth

May either refer to the rare earth elements, or oxides of the rare earth elements (i.e. One or more of the rare earth elements combined with oxygen).

Reflection

The event occurring when light hits a material and bounces off of it. There are different degrees of reflection, the strongest being light rays bouncing off a smooth, flat, polished surface, such as a mirror, where they bounce off and form a reversed image on the surface. Polished Hematite is a great example describing a reflective mineral, where in some cases it produces a mirror-like reflection.

Reflective

Giving off or having to do with reflection.

Refract

Bend.

Refraction

The bending of white light upon entering a new dimension, such as from air to water or from air to a crystal, and splitting the white light into the colors of the spectrum.

Refractive index

The amount of refraction that takes place in a particular substance, which is a direct connection to the speed of light in that substance. The higher the refractive index, the greater the amount of dispersion, which increase the brilliance of a material. The refractive indices of gems are measured with a refractometer. Gems can be identified by measuring their refractive indices.

Refractometer

Instrument that measures the refraction of light when it enters through a different dimension, such as from air to water or from air into a crystal.

Reniform

Aggregate describing smooth, rounded, kidney-like agglomerations. Rounded agglomerations of reniform aggregates are larger than botryoidal agglomerations but smaller than mammilary agglomerations.

Repeated twinning

Form of twinning where two or more crystals form a repeated pattern. Examples: cassiterite twin, rutile twin, plagioclase twin, and fishtail twin.

Replace

Having some or all the atoms being exchanged for atoms of a similar element. See replacement.

Replacement

The process of one mineral taking the place of another mineral or material, with one or two atoms per molecule in the structure being exchanged with a different one with similar characteristics, thus creating a new mineral that retains the shape of the first mineral. (See Pseudomorph for more information.) Also refers to one element of a mineral to partially or fully substitute itself to another element.

Resinous (luster)

Luster describing yellow, dark orange, or brown minerals with slightly high refractive indices -- honey like, but not necessarily the same color.

Resistance

The unyielding of material to destructive acids and tough environments.

Reticulated

Aggregate composed of long crystals in a netting-like form, where all the crystals crisscross each other. (i.e. Crocoite)

Rhombohedral cleavage

Type of cleavage exhibited on minerals that crystallize in the hexagonal system and as rhombohedrons, in which small rhombohedrons cleave off of the existing rhombohedron. Example = Calcite.

Rhombohedron

Six sided polyhedron with each side shaped as a rhombus.
A mixture of a rhombohedron and scalenohedron also exists. It is sometimes called a rhomb-scalenohedron.

Rift

Crack in a rock. On a large scale it refers to the separation of plates on fault areas, forming a depression in the earth.

Rock

An indefinite mixture of naturally occurring substances, mainly minerals. Its composition may vary in containment of minerals and organic substances, and are never exact.

Rock crystal

Transparent, colorless, crystal of Quartz.

Rock shop

Commercial store where minerals and rough gemstones can be purchased.

Rock show

Periodical show in a large, usually indoor arena where mineral and gem dealers come to sell their stock at wholesale prices.

Rosette

Mineral with concentric aggregates resembling rose flowers.

Rough

Without any crystal faces. In regard to gemstones it refers to unfaceted material.

Rust

Powdery or crusty, brownish-yellow coating of iron oxides commonly see on iron materials after they come in contact with water.

Rusting

Developing a coat of rust on the surface, or containing a rust coating from a previous rusting.

Rutilated

Containing Rutile impurities.

Rutile twin

Two or more crystals that twin in a repeated pattern ("repeated twinning") as depicted in the figure below. Named after the mineral Rutile, which most frequently exhibits this form of twinning. If there are six repeated crystals (i.e. another three crystals are added to the figure below in the same pattern), the agglomerate forms a circular structure, known as a sixling.

Saccharoidal

Composed of tiny, equidimensional crystals that resemble grains of sugar.

Saline

Containing salt.

Saline deposit

Mineral environment containing minerals formed through partial or full evaporation of a mineral-rich, saline lake.

Saline lake

Mineral-rich lake found in arid regions that contains a heavy salt content, and frequently contains evaporite minerals which form as the lake evaporates. Saline lakes may fully evaporate, in which they become dry lakes.

Salinity

Amount of salt present.

Salt

1) Compound formed when a metal partially or fully takes the place of hydrogen in an acid.
2) Halite.

Salt dome

Separated fold in an anticline which is plugged up with a mass of salt.

Sand

Accumulation of small grains of rock, usually found at beaches and in desert regions.

Scalenohedral

Shaped as a scalenohedron.

Scalenohedron

Six sided polyhedron, similar to a bipyramidal hexagon, but the adjoining area at the center is diagonal between every side as opposed to being level. Other modifications might be present.
A mixture of a rhombohedron and scalenohedron also exists. It is sometimes called a rhomb-scalenohedron.

Scales

Aggregate of small, flattened, overlapping crystals, as seen in fish scales.

Scaly

Describing an aggregate of scales.

Scepter

Crystal deformity where the top part of a prismatic crystal protrudes and is wider than the rest of the crystal.

Sclerometer

Instrument used to determine the absolute hardness of a mineral. A mineral is locked into position , and a Diamond pin pushes down and slightly scratched the mineral. The amount of pressure needed for the Diamond pin to be pushed down to scratch the mineral determines its hardness. (See Hardness in mineral properties for more information.)

Schiller

Color reflections or color "flashes" present in a mineral. The individual color flashes of Opal are known as "schillers".

Schist

Type of metamorphic rock made up of parallel layers of flaky, micaceous minerals.

Secondary

Altered to create a new mineral. (See "secondary mineral")

Secondary mineral

Mineral altered to a new form after undergoing a chemical change. An example is a mineral in the oxidation zone that transformed into another mineral through weathering.

Sectile

Able to be cut into by a knife or other sharp object. A form of tenacity.

Selenides (subgroup)

Group of sulfides the contain one or more true metals combined with the semi-metal selenium.

Selvage

The area of the point of contact between a vein and the surrounding rock

Sediment

Eroded pieces of rock that get carried away in streams and form deposits in them, which eventually cement together and form sedimentary rock.

Sedimentary rock

Rock formed by the weathering of substances; forming layers from accumulation of minerals and organic substances.

Sedimentation process

Process in which sediment transforms into sedimentary rock.

Semi-metal

Element or alloy exhibiting certain properties of the metals and certain properties of the non-metals. They are opaque with a metallic luster, and combine with other elements to form minerals as metals do. In all other aspects, they act like non-metals.

Semi-metallic

Having the attributes of a semi-metal.

Semi-metallic elements (subgroup)

The semi-metallic elements are minerals belonging to the native elements group and are composed of pure semi-metallic elements or semi-metallic alloys. They are opaque with a metallic luster, and combine with other elements to form minerals (i.e. Arsenides, Tellurides) the same way the metallic elements do.

Semi-precious stone

Gem or gemstone used in jewelry but lacks in one or more property (such as luster, hardness, and rarity) that would make it into a precious gemstone.

Series

It is very hard to define a mineral series in writing. The following is a bit confusing but is the simplest way to put it in words.

A series is a mineral group consisting of minerals that have a nearly identical crystal structure. The only difference between the minerals in a series is the type of elements contained in the crystal structure of the mineral. This means that there must be at least two defined minerals in a series: one mineral containing one particular element, and one mineral containing a different element. These two minerals are known as the end members, or anchor members. In-between the end members minerals as a combination of both anchor members exist. Sometimes, these intermediary minerals are given names. An example of this is a mineral series known as the Olivine group. In the Olivine group, the two end members are Forsterite and Fayalite. Forsterite and Fayalite have a crystal structure that is identical, except that Forsterite is composed of magnesium (Mg), and Fayalite is composed of iron (Fe).

Forsterite	Mg2SiO4
Fayalite	Fe2SiO4

Another member of the Olivine group is Chrysolite. Chrysolite is an intermediary member, so it has a varying amount of magnesium and iron.

Chrysolite

(Mg,Fe)2SiO4

The amount of magnesium to iron in Chrysolite is not defined, and can range from 1:99 to 99:1. However, in certain mineral series, intermediary members are given names based on defined percentages of the elements they contain. For example, if an intermediary member contained 20% Mg and 80% Fe, it might be classified as a different mineral than 80% Mg and 20% Fe.

 

Some mineral series, such as the Spinel series, contain more than two end members. This means there can be many different combinations of intermediary members.

Spinel Series (anchor members)
Common Spinel	MgAl2O
Gahnite	ZnAl2O
Hercynite	FeAl2O
Galaxite	MnAl2O

Some possible combinations might be:

(Mg,Zn)Al2O

(Mg,Fe)Al2O

(Zn,Fe)Al2O

(Mg,Zn,Fe)Al2O

(Mg,Fe,Mn)Al2O

Most of these possible combinations do not have names, but a few, such as Gahnospinel,(Mg,Zn)Al₂O₄, do.

 

In a few complex mineral series, such as Garnet, more than one element may be replaced by another.

Garnet member
Pyrope	Mg3Al2Si3O12
Almandine	Fe2+3Al2Si3O12
Spessartine	Mn3Al2Si3O12
Grossular	Ca3Al2Si3O12
Andradite	Ca3Fe3+2Si3O12
Uvarovite	Ca3Cr2Si3O12

The mineral Pyrope and the mineral Uvarovite contain TWO elements that are different, and yet they are still in the same series, since they are identical in crystal structure, and can (theoretically) form intermediary members.
In large series such as Garnet, nearly hundreds of possible intermediary combinations can exist. However, this number is greatly restricted since the laws of chemistry only allow certain members to combine with others.

A mineral series may contain a member with the same name. For example, the Skutterudite series contains three minerals, and the name of one of them is Skutterudite.

Setting

The proper placement of a gem on an ornament, or the ordained area for the placement of a gem.

Shaft

A long, narrow passage in underground mines created for the extraction of valuable material.

Shortwave

A wavelength of ultraviolet light. Many minerals display fluorescence when exposed to shortwave ultraviolet light.

Silica

Radical of silicon and oxygen. Also term for any material composed of only silicon and oxygen (and can include water), such as Quartz, Chalcedony, and Opal.

Silicafied

Having been transformed into silica. An example is petrified wood, or wood that was completely replaced by silica.

Silica gel

Chemical that absorbs moisture from air.

Silica group

Group of silicate minerals (tectosilicates) composed only of silica (silicon dioxide -- SiO2 ). The minerals in this group are all the varieties of Quartz, Chalcedony, Opal, Tridymite, and Cristobalite (and a few rarer forms of silicon dioxide). Although Opal contains water in addition to silica, it is nevertheless in the silica group. The Silica Group is also known as the Quartz Group. See Polymorphs of SiO2 for more detailed information on the minerals in the silica group.

Silicates (group)

Group of minerals containing various amounts of the elements silicon and oxygen. For more information, see "All About the Silicates Group".

Sill

Horizontal "sheet" of igneous rock interjected between rock that was there before the igneous rock. A famous example is the Palisades Sill right near on the western side of the Hudson River right near New York City.

Silky (luster)

Luster of minerals that have a very fine fibrous structure, causing it to display similar optical properties to silk cloth.

Silt

Accumulation of very small grains of rock, finer than sand

Simple oxides (subgroup)

Compounds of metallic elements combined with oxygen. The simple oxides are a subgroup of the oxide group.

Simple sulfides (subgroup)

Group of sulfides that contain one or more true metals combined with sulfur.

Sinter

Mineral deposit formed by mineral-rich spring water which comes to the surface and loses its capacity to withhold the dissolved mineral, thereby depositing it and forming a growing mound.

Skeletal

Aggregate synonymous with dendritic. Some references note a difference in that skeletal is a single, tree-like entity, whereas dendritic is the same pattern implanted in rock.

Slab

Thick slice cut out of a solid mineral; a portion of the mineral.

Slag

Glassy substance formed as a by-product from the smelting of metallic ores.

Smelt

To melt ores for the extraction of a valuable metal they contain.

Soil

The top layer of the earth's surface, which is composed of tiny rock particles mixed together with decaying organic matter.

Solid solution

A series formed when there are minerals in-between two isomorphous minerals. An example is the Plagioclase Feldspar group, which contains Albite and Anorthite as the anchor minerals, and minerals varying in amounts of those two minerals. Solid solutions may also be composed of minerals with a definite structure where one element may be minutely replace by another, but is too insignificant to be mentioned in the structure of that mineral.

Solidify

Transforming from a liquid state to a solid state.

Solubility

The condition of being soluble.

Soluble

Able to be dissolved.

Sorosilicates

Group of silicate minerals that are composed of groups of two tetrahedrons where one of the oxygen atoms is shared by both tetrahedrons; the other three oxygen atoms are not shared by another tetrahedron

Spar

Any non-metallic, lightly colored mineral with good cleavage.

Specific gravity

The weight ratio of a mineral due to the density of the atomical arrangement and the heaviness of the elements it contains. (See Specific Gravity in mineral properties for more information.)

Specimen

A mineral or rock of interest to collectors or scientists.

Spectral

Having to do with the spectrum.

Spectrograph

Device used to observe and record spectral changes.

Spectrophotometer

Device used to to measure the brightness of different portions of the spectrum.

Spectroscope

Device, such as a prism, which refracts white light and produces a spectrum which can be observed.

Spectroscopy

The science and study of the spectrum.

Spectrum

Composite of white light, which are the colors of the human eye can see. The spectrum is composed of the colors red, orange, yellow, green, blue, indigo, and violet in that order.

Sphenoidal

Prism with wedge shaped corners, formed from a bipyramid that did not properly develop.

Spherulite

Any mineral composed of spherulitic aggregates.

Spherulitic

Aggregate consisting of rounded, ball like structures composed of radiating crystals. A famous example is Wavellite.

Spinel twin

Form of contact twinning, in which two octahedral crystals that twin at the base, as depicted in the figure below. Named after the mineral Spinel, which most frequently exhibits this form of twinning.

Splintery

Crystal habit where crystals are long, thin, straight splinters. "Splintery" also describes an aggregate composed of such crystals.

Splintery (fracture)

Fracture forming elongated splinters. All fibrous minerals fall into this category.

Stable

Able to withstand normal environments without disintegrating, unlike unstable minerals.

Stalactites

Icicle-like formations on the roof of caverns created when mineral-rich water drips down from the roof and the dissolved mineral accumulates into the icicle-like formation. May be confused with stalagmites, which are tall-domed formations on the bottom of caverns built up from the mineral-rich water depositing the dissolved mineral on the floor.

Stalactitic

Aggregate composed of long, icicle-like formations, like stalactites.

Stalagmites

Tall-domed formations on the bottom of caverns built up from mineral-rich water that deposits the dissolved mineral on the floor, growing upwards. May be confused with stalactites, which are icicle-like formations on the ceiling of caverns created from the mineral-rich water dripping down, and the dissolved mineral accumulates into the formation.

Stalagmitic

Shaped as a stalagmite, as a tall-domed structure. Many times confused with the term stalactitic, and many times substituted for that word.

Staurolite twin

Form of penetration twinning where two monoclinic crystals form interpenetrating twins at 90º, forming a cross, as depicted in the figure below. Named after the mineral Staurolite, which most frequently exhibits this form of twinning. (Staurolite crystals may also twin at angles other than 90º, such as 60º.)

Stock

Body of exposed igneous rock that intruded a layer of underground sedimentary rock, and is smaller than around 40 square miles (around 100 square kilometers).

Stone

Small piece of rock; may or may not refer to ornamental material.

Streak

The color of the powder of a mineral. Streak can be tested by rubbing a mineral against a hard, white, porcelain object (streak plate), and the "streak" that remains on the swiped area is the color of the streak of the mineral. (See Streak in mineral properties for more information.)

Streak plate

Unglazed piece of porcelain, such as a tile, used to test a mineral's streak.

Striated

Exhibiting tiny parallel lines (or grooves -- depending if it sounds better that the cup is half empty rather than half full) on crystal faces.

Striations

Tiny, parallel lines seen on some crystal faces.

Stratification

The behavior of having mineral sediments embedded in sedimentary rock.

Stratum

Layer of sedimentary rock compromised of one rock type. Layers of strata containing different rocks may form parallel bands of different rock.

Structural

Relating to the structure of a mineral.

Structure (minerals)

The form of a mineral based on the way its molecules are arranged. Also short for Chemical structure.

Structure (rocks)

Features exhibited in rock portions, for example flow banding and bedding.

Stubby

Short and fat. Stubby crystals are short and fat.

Stud

Small jewel placed on an ornament, such as an earring. May also refer to the verb form of setting with a stud. Sometimes additionally refers to a cable that holds together a metallic ornamental chain, such as a necklace.

Subconchoidal (fracture)

Mineral fracture that falls somewhere between conchoidal and even; being smooth with irregularly rounded corners.

Subgroup

The next level of mineral classification after categorization in groups (definition 3).

Sublimate

To go from a solid state directly to a gaseous state without becoming liquid.

Sublimation

The act of sublimating.

Submetallic (luster)

Luster of opaque to nearly opaque minerals with very good reflective properties.

Sulfides (group)

Group of minerals that contains one or more metallic element in addition to the sulfate radical (SO4). All sulfates are transparent to translucent and soft. Most are heavy and lightly colored, and some are soluble in water. Rarer sulfates exist containing substitutions for the sulfate radical, such as the chromates, where it is replaced by a chromate radical (CrO4). The sulfates can be divided into the Hydrous sulfates and the Anhydrous sulfates. The chromates are usually classified as a sub-group of the sulfates.

Sulfosalts (subgroup)

Group of sulfides that contain one or more true metals, sulfur, and either the semi-metals antimony, arsenic, or bismuth. They are generally soft, have a metallic luster, and are heavy and uncommon.

Sulfuric acid

(H2SO4) Corrosive, oily, acid used for manufacturing chemicals, medicine, paints, detergents, and explosives. It is a very destructive liquid and will destroy many minerals.

Surrounding rock

The rock surrounding a vein or embedded crystal.

Swallowtail twin

Form of penetration twinning in which two monoclinic crystals twin to form a v-shaped model, as depicted in the figure below. This form of twinning is most frequently seen on the mineral Gypsum.

Syenite

Igneous rock containing alkali feldspars. Contains mica and pyroxene minerals.

Symmetrical

Containing perfect symmetry.

Symmetry

Synthetic gem

Gem created by man using molten chemicals to solidify and form the gem.

Tabular

Crystal habit describing a flat, tough, usually four sided crystal.

Tarnish

Property in certain minerals that cause them to discolor when placed in certain environments. A famous example is Silver, which develops a yellowish-black coating on its surface when exposed to hydrogen sulfide (H2S), which is present in small quantities in the atmosphere. See also oxidize.

Tectosilicates (subgroup)

Group of silicate minerals that have all four oxygen atoms from each tetrahedron share itself with an adjacent tetrahedron.

Tellurides (subgroup)

Group of sulfides that contain one or more true metals combined with the semi-metal tellurium.

Tenacity

The reaction of a mineral to certain stress. (See Tenacity in mineral properties for more information.)

Termination

The end of a crystal face, usually referring to its base.

Tetragonal (crystal system)

Any mineral that falls under the following specifications belongs to the tetragonal crystal system: Three axes, two are equal in length, one is unequal. All three axes are at 90° to each other.

Tetrahedral (shape)

Exhibiting the shape of a tetrahedron.

Tetrahedron (silicates)

The building block of all silicates. It is composed of one silicon atom equidimensionally placed around four oxygen atoms. The groups of silicates are classified based on how the tetrahedrons join together.

Texture

The feel and appearance of a mineral, such as color, crystal form, and luster.

Thermoluminescence

Phenomenon exhibited in certain minerals that they give off light, like a glow, when heated. Can only be observed in a dark area.

Thermoluminescent

Emitting light after being heated.

Transformation

The alteration of one mineral to another.

Transparency

The quality of how a mineral lets light through.

Transparent

Describing an object that is able to transmit light through it, and if placed in front of another object, that object can be seen as if there is nothing interfering.

Translucent

Describing something that is able to transmit light but not fully. Objects can be seen through a translucent object, but they will be unclear.

Trap rock

Rock formed from solidified lava, which usually contains pockets holding crystals.

Triboluminescence

Property exhibit in certain minerals which causes them to give off orange or yellow "flashes" when sawed or struck.

Triclinic (crystal system)

Any mineral that falls under the following specifications belongs to the triclinic crystal system: Three axes, all of them are unequal in length, none of them are right angles to each other.

Trigonal (crystal system)

The trigonal crystal system is really part of the hexagonal crystal system and only has one minute difference (in that true hexagonal minerals have sixfold symmetry, whereas trigonal minerals have threefold symmetry). The trigonal crystal system, like the hexagonal crystal system, has: four axes, three are equal in length and lie at an angle of 120° from each other. The fourth is either longer or shorter but must be at a right angle toward the other corners.

Trilling

Intergrowth of three orthorhombic crystals that twin at the center and form a hexagonal shaped crystal

Triplet

Gem composed of three cross-sections pasted together. The top portion, which is the portion that is visible, consists of a naturally occurring, valuable gem, while the bottom two portions are low-grade material, such as glass or rock crystal. See also doublet.

Trisoctahedron

24 sided polyhedron; all sides are equidimensional. Minerals shaped as trisoctahedrons belong to the isometric system. Plural = Trisoctahedra or Trisoctahedrons

Trunk

An individual portion of a vein.

Tube test

Complex, scientific test which is conducted to identify a mineral. A mineral is crushed and its powder is placed in an open or closed tube, which is then heated; the reaction of the powder in the tube to the heat acts as a guide for identifying the presence of certain metals in the mineral.

Tuberose

Aggregate composed of large, oval, "bubbles" protruding from a surface. Similar to pisolitic, but "bubbles" are protruding instead of slightly rounded. (i.e. Aragonite)

Tufacious spring

Spring which produces tufa.

Tuff

Porous igneous rock composed of volcanic ash compacted together.

Tumbled

Having been placed in a tumbler, which smoothes out and enhances the luster of a mineral. Rocks and minerals may be naturally tumbled, if they were smoothed and rounded from having been in a fast-flowing stream or river for a long period of time.

Tumbler

Machine that smoothes the surfaces and increases the luster of minerals by "tumbling" them together in a cylindrical container that rotates, causing them to continually bang into one another.

Tungstates

Group of minerals that are compounds of one or more metallic elements and the tungstate radical (WO4). These minerals are very heavy, soft, and brittle. The tungstates are usually categorized with the chemically related molybdates, in which they can be partially replaced by. The tungstates are also known as the wolframates.

Twin

Two or more crystals that intergrow in a specific method. May also be used in verb form, as "Two crystals may twin to form a cross." See Crystals in mineral properties for more information.

Twinning

Tendency of some crystals to intergrow in a distinct way or form specific, repeated patterns. See penetration twinning, repeated twinning, and contact twinning.

Ultrabasic

Describing an igneous rock containing magnesium and iron with a very low percentage of silica (less than 45%)

Ultraviolet

Pertaining to ultraviolet light.

Ultraviolet light

Type of electromagnetic radiation which cannot be seen by humans.

Uneven (fracture)

Fracture that leaves a rough or irregular surface.

Unstable

Mineral with a crystal structure that is unable to exist at the current temperature. When certain minerals become unstable, they transform into their paramorphs. Also refers to radioactive minerals which cannot withstand their composition and begin to decay.

U.V.

Abbreviation of ultraviolet

Vanadates (group)

Group of minerals that are compounds of one or more metallic element associated with the vanadate radical (VO4). The vanadates, together with the related arsenates, are classified in the phosphate group. Most vanadates are heavy, and none are hard. They are usually brittle and occur in small crystals or compact aggregates.

Variable formula

Chemical formula of a mineral that includes all common elemental replacements. For example, the mineral Adamite - Zn2(AsO4)(OH), frequently contains part of the zinc (Zn) replaced by copper (Cu) or cobalt (Co); thus its variable formula is (Zn,Cu,Co)2(AsO4)(OH).

Variety

Titled variable of a mineral, characterized by a special property not present in other members of that same mineral (such as color, location, chemical additive, crystal or aggregate, etc.)

Vein

A long, thin section of a mineral, usually in fibrous form, embedded in rock [called the surrounding rock]. Usually caused by the filling in of the mineral in open spaces or cracks in the rock.

Vent

Area in the earth's crust where hot gases or lava escape from.

Venting

The escaping of hot gases or lava from the crust of the earth.

Verneuil process

The course of creating synthetic gems from molten solutions and letting them form into boules.

Vitreous (luster)

Luster describing minerals with reflective properties similar to that of glass.

Volatilize

Disappear as fumes.

Volcanic

Having originated from a volcano.

Volcanic rock

Rock formed as a direct result of a volcano (i.e. the solidifying of magma).

Volcanism

The volcanic venting effects associated with volcanoes: lava flows, hot springs, smoking, and fumaroles.

Volcano

A vent in the crust of the earth which spits out molten rock (lava), ash, and gases.

Vug

Cavity in rock that is lined with long, slender crystals. A vug forms when air pockets form in cooling magma and allow crystals to form in the hollow area.

Waxy (luster)

Luster of a mineral in which it appears to be coated by a layer of wax.

Weathered

Having gone through the process of weathering.

Weathering

The passive act of a mineral that was exposed from the earth and was chemically affected in one way or another, either by air, water, pressure, or wind.

Wheat sheaf

Aggregate of compact bundles of crystals, slightly radiating and thicker at the top and the bottom than in the center. Appears in the shape of an hourglass.

White light

Electromagnetic radiation that the eye can detect, which is known as light.

Widmanstaetten lines

Etched crystal faces that are seen on some polished meteorites.

Wires

The individual wires of a wiry aggregate.

Wiry

Aggregate composed of long, slender, curvy, interwoven wires.

Xenolith

Rock fragment or mineral found in igneous rock that is not of igneous origins.

X-ray

Electromagnetic radiation that has a very short wavelength, emitted from a substance when it is attacked by a line of electrons in a vacuum. Machines are able to produce this, and they are known as x-ray machines. They are useful in geology and mineralogy for the study of the chemical constituents of rocks and minerals.

Zeolite

Individual member of the zeolite group.

Zeolite group

Group of around 30 minerals that are related in structure, habits, and occurrence. The zeolites are hydrous aluminum silicates that contain sodium or calcium, which can replace one another. These minerals are usually found in zeolite deposits, which are of igneous origin. For more information, see "All About the Zeolites Group". The zeolite group is in the tectosilicates subdivision of the silicates group.

Zoning

The distinction of particular rock material from surrounding rock material.