C

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