Welcome to the Mineral and precious stone glossary

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z  All terms

A

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.