Glossary of Rock and Mineral Terms - C

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cabochon

Gem or stone without facets that is highly polished and has smooth, rounded edges. (See our How To Cab online guide)

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

caldera

A vast depression at the top of a volcanic cone, formed when an eruption substantially empties the reservoir of magma beneath the cone's summit. Eventually the summit collapses inward, creating a caldera. A caldera may be more than 15 kilometers in diameter and more than 1000 meters deep.

caliche

A white soil horizon consisting of calcium carbonate, typical of arid and semi-arid areas. Brief heavy rains dissolve calcium carbonate in the upper layers of soil and transport it downward; the rainwater then evaporates rapidly, leaving the calcium carbonate to form a new, solid layer of soil.

cameo

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

capacity

The ability of a given stream to carry sediment, measured as the maximum quantity it can transport past a given point on the channel bank in a given amount of time. See also competence.

capillary

Aggregate composed of tiny, thin, straight, long crystal strands; hair like.

capillary fringe

The lowest part of the zone of aeration, marked by the rising of water from the water table due to the attraction of the water molecules to mineral surfaces and other molecules, and to pressure from the zone of saturation below.

carbon-14 dating

A form of isotope dating that relies on the 5730-year half-life of radioactive carbon-14, which decays into nitrogen-14, to determine the age of rocks in which carbon-14 is present. Carbon-14 dating is used for rocks from 100 to 100,000 years old.

carbonate

One of several minerals containing one central carbon atom with strong covalent bonds to three oxygen atoms and typically having ionic bonds to one or more positive ions.

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.

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 into Calcite Group and  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 as depicted in the figure below.

carving

Ornamental figure, such as a stone lion, carved out of a rock or mineral. A piece of stone formed this way is described as carved.

cassiterite twin

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

catastrophism

The hypothesis that a series of immense, brief, worldwide upheavals changed the Earth's crust greatly and can account for the development of mountains, valleys, and other features of the Earth. See also uniformitarianism.

cation

Neutral atom that loses an electron and becomes positively charged.

cats 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 (cavern)

A naturally formed opening beneath the surface of the Earth, generally formed by dissolution of carbonate bedrock. Caves may also form by erosion of coastal bedrock, partial melting of glaciers, or solidification of lava into hollow tubes.

cavity

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

 

cementation

The diagenetic process by which sediment grains are bound together by precipitated minerals originally dissolved during the chemical weathering of preexisting rocks.

cementation zone

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

Cenozoic Era

The latest era of the Phanerozoic Eon, following the Mesozoic Era and continuing to the present time, and marked by the presence of a wide variety of mammals, including the first hominids.

chemical sediment

Sediment that is composed of previously dissolved minerals that have either precipitated from evaporated water or been extracted from water by living organisms and deposited when the organisms died or discarded their shells.

chemical weathering

The process by which chemical reactions alter the chemical composition of rocks and minerals that are unstable at the Earth's surface and convert them into more stable substances; weathering that changes the chemical makeup of a rock or mineral. See also mechanical weathering.

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. Chatoyant is the ability to exhibit chatoyancy.

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.

chemical structure

The atomic arrangement of a substance.

chemistry

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

chert

A member of a group of sedimentary rocks that consist primarily of microscopic silica crystals. Chert may be either organic or inorganic, but the most common forms are inorganic.

chondrules

Small nuggets of rocky material that exist in certain meteorites. These droplets of matter are believed to have condensed from our solar system's original nebula about five billion years ago. Their primary element is iron.

chromate (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, as depicted in the figure below.

cinder cone

A pyroclastic cone composed primarily of cinders.

cirque

A deep, semi-circular basin eroded out of a mountain by an alpine glacier.

cirque glacier

A small alpine glacier that forms inside a cirque, typ-ically near the head of a valley.

clast

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

clastic

Being or pertaining to a sedimentary rock composed primarily from fragments of preexisting rocks or fossils.

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 tendency of certain minerals to break along distinct planes in their crystal structures where the bonds are weakest. Cleavage is tested by striking or hammering a mineral, and is classified by the number of surfaces it produces and the angles between adjacent surfaces.

cleavage angle

The angle, or side, that exhibits or has exhibited cleavage.

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.

clinopinicoid

Describing an elongated crystal with a steep, slanted angle towards its top at the base. Minerals shaped this way are clinopinicoidal.

cluster

Dense agglomeration of crystals.

coal

A member of a group of easily combustible, organic sedimentary rocks composed mostly of plant remains and containing a high proportion of carbon.

coast

The area of dry land that borders on a body of water.

coating

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

col

A high mountain pass that forms when part of an arête erodes.

color

The characteristic color or colors of a mineral.

columnar

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

compaction

The diagenetic process by which the volume or thickness of sediment is reduced due to pressure from overlying layers of sediment.

competence

The ability of a given stream to carry sediment, measured as the diameter of the largest particle that the stream can transport. See also capacity.

composite cone

See stratovolcano.

composition

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

compound

An electrically neutral substance that consists of two or more elements combined in specific, constant proportions. A compound typically has physical characteristics different from those of its constituent elements.

compression

Stress that reduces the volume or length of a rock, as that produced by the convergence of plate margins.

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 fracture

Mineral fracture in which the indentation resembles a shell.

concretion

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

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.

cone of depression

An area in a water table along which water has descended into a well to replace water drawn out, leaving a gap shaped like an inverted cone.

conglomerate

A clastic rock composed of particles more than 2 millimeters in diameter and marked by the roundness of its component grains and rock fragments.

contact metamorphism

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.

continental collision

The convergence of two continental plates, resulting in the formation of mountain ranges.

continental drift

The hypothesis, proposed by Alfred Wegener, that today's continents broke off from a single supercontinent and then plowed through the ocean floors into their present positions. This explanation of the shapes and locations of Earth's current continents evolved into the theory of plate tectonics.

continental ice sheet

An unconfined glacier that covers much or all of a continent.

continental platform

Continental platforms are the regions adjacent to and surrounding the continental shields. They are typically a relatively thin veneer of sedimentary rock that buries the edges of the shields.

continental shield

Broad areas of exposed ancient crystalline rocks in the cores of the Earth's continents. These rocks are typically the oldest on the continentes, many more than 2.5 billion years old.

convection cell

The cycle of movement in the asthenosphere that causes the plates of the lithosphere to move. Heated material in the asthenosphere becomes less dense and rises toward the solid lithosphere, through which it cannot rise further. It therefore begins to move horizontally, dragging the lithosphere along with it and pushing forward the cooler, denser material in its path. The cooler material eventually sinks down lower into the mantle, becoming heated there and rising up again, continuing the cycle. See also plate tectonics.

convergence

The coming together of two lithospheric plates. Convergence causes subduction when one or both plates is oceanic, and mountain formation when both plates are continental. See also divergence.

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, consisting primarily of pure metals such as iron and nickel. The core is the densest layer of the Earth, and is divided into the outer core, which is believed to be liquid, and the inner core, which is believed to be solid. See also crust and mantle.

correlation

The process of determining that two or more geographically distant rocks or rock strata originated in the same time period.

covalent bond

The combination of two or more atoms by sharing electrons so as to achieve chemical stability under the octet rule. Atoms that form covalent bonds generally have outer energy levels containing three, four, or five electrons. Covalent bonds are generally stronger than other bonds.

coxcomb

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

crater

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

craton

The segment of the Earth's continents that have remained tectonically stable and relatively earthquake-free for a vast period of time. The craton is composed of the continental shield and the surrounding continental platform.

crazing

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

creep

The slowest form of mass movement, measured in millimeters or centimeters per year and occurring on virtually all slopes. cross bed A bed made up of particles dropped from a moving current, as of wind or water, and marked by a downward slope that indicates the direction of the current that deposited them.

crust

1. The outermost layer of the Earth, consisting of relatively low-density rocks. See also core and mantle.
2. A disorganized, crusty, mineral coating that can be thin or thick.
3. Type of aggregate.

crusty

Aggregate of a crust coating on a rock or mineral.

cryptocrystalline

Composed of tiny, microscopic crystals.

crystal

A mineral in which the systematic internal arrangement of atoms is outwardly reflected as a latticework of repeated three-dimensional units that form a geometric solid with a surface consisting of symmetrical planes.

crystal angle

The sum of the angles on a crystal edge that are characteristic to a crystal set.

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.

crystal face

The side of a crystal. The number of faces varies with the crystal's structure.

crystal form

The shape and habit of a particular crystal.

crystal habit

The habitual form that a mineral forms it crystallizes.

crystal lattice

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

crystal structure

1. The geometric pattern created by the systematic internal arrangement of atoms in a mineral. 
2. The systematic internal arrangement of atoms in a mineral.

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.

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.

crystallized

Containing a crystal form. May also be referred to molten rock that solidifies and forms a crystal shape.

crystallography

The science and study of crystal structure. Person who studies crystallography is a crystallographer.

Ct.

Abbreviation for carat

cube

Six sided polyhedron; all sides are equi-dimensional and bisect at 90º. Minerals shaped as cubes belong to the isometric system.

cubic crystal system

Synonym of 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.

cut

1. n A description of the type of facet.
2. v Meaning faceted.

cyclosilicates group

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.

cymophone effect

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