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Terms are defined within the contexts of mineralogy, crystallography, mineral collecting, geology, gemology, paleontology, or mining.


ACCESSORY ELEMENT: An elemental constituent of a mineral that is present in such small amounts that it is disregarded in the mineral’s chemical formula.

ACCESSORY MINERAL: A mineral constituent of a rock that is present in such small amounts that it is disregarded in definition, chemistry, and classification of the rock.

ACICULAR: Having a needle-shaped crystal habit.

ACID (OR ACIDIC) ROCK: An igneous rock consisting of more than 66 percent free or combined silica. Examples: Granite, rhyolite.

ADAMANTINE: Having a bright luster similar to that of diamond.

ADIT: A horizontal passage connecting the surface and the workings of an underground mine.

ADULARESCENCE: The display in certain gemstones of a milky, bluish-white, surface sheen. Example: The adularia (moonstone) variety of orthoclase.

AGGREGATE: An assemblage of individual crystals.

ALLOCHROMATIC: Literally “other-colored”; refers to minerals with colors that are caused by trace amounts of nonessential elements.

ALLOY: A metallic material made by fusing together two or more metals or a metal and a nonmetal. Example: Bronze, an alloy of copper and tin.

ALLUVIUM: Unconsolidated bits of rocks and minerals in secondary deposits that form from the action of running water.

ALTERATION: The chemical or physical changes that occur to a rock or mineral after its original formation that can result in textural changes to the rock or cause the formation of new minerals. Example: The alteration of azurite into malachite.

AMALGAM: An alloy of mercury with another metal.

AMORPHOUS: Literally “without form”; describes minerals that lack definite crystal structure. Example: Obsidian, a high-silica, non-crystalline, natural volcanic glass.

AMYGDALOIDAL ROCK: A volcanic rock characterized by numerous vesicles that are filled with secondary minerals.

AMYGDULE: A vesicle in volcanic rock that is filled with secondary minerals. Example: Zeolite-filled amygdules in basalt.

ANHEDRAL: Poorly developed crystal shape.

ANHYDROUS: Literally “without water”; a mineral containing no water molecules in its crystal structure.

ANION: An atom that has gained one or more electrons: a negatively charged ion.

ANIONIC CHARGE: The total negative charge of an anion or anionic radical. Example: The anionic -2 charge of the carbonate radical (CO3)2-.

ANTHRACITE COAL: A type of coal, called “hard coal,” containing 86 to 97 percent carbon that formed from high-grade metamorphism of bituminous coal.

APHANITIC ROCK: A rock consisting of crystalline constituents too small to be seen by the unaided eye.

ARBORESCENT: Dendritic; having a tree-like or fern-like shape. Example: “Scenic” or “moss” agate.

ARGENTIFEROUS: Containing silver.

ARGILLACEOUS: Consisting largely of clay.

ARSENATE: A mineral consisting of one or more metals in combination with the arsenate radical (AsO4)3-. Example: Adamite [basic zinc arsenate, Zn2(AsO4)(OH)].

ARSENIDE: A mineral consisting of one or more metals in combination with the semimetal arsenic. Example: Skutterudite [cobalt arsenide, CoAs3].

ASTERISM: The ability of certain translucent crystals to display 4-, 6-, or 12-rayed “stars” due to the reflection of light from inclusions of aligned, microscopic rutile crystals. Examples: Star sapphire and star ruby.

ATOM: The smallest particle of an element that can exist either alone or in combination. AURIFEROUS: Containing gold.

AVENTURESCENCE: The glittering appearance of a crystal caused by tiny inclusions of minerals, such as the mica-group minerals, with highly reflective surfaces. Example: The aventurine variety of quartz.

AXIS: In crystallography, a theoretical line drawn through the center of a three-dimensional object, such as a crystal, that defines its symmetry; the arrangement and relative lengths of three or four axes determine the crystal system.


BASIC MINERAL: A mineral containing the hydroxyl radical or ion (OH)1-. Example: Adamite [basic zinc arsenate, Zn2(AsO4)(OH)].

BASIC ROCK: A dark-colored, igneous rock that is low in silica and rich in ferro-magnesian minerals. Examples: Basalt, gabbro.

BATHOLITH: In geology, a massive, irregularly shaped, igneous intrusion, usually greater than 40 square miles in area, that formed deep within the Earth’s crust and was later exposed by erosion.

BEDROCK: The solid rock formations directly underlying surface deposits of unconsolidated soil, sand, and gravel.

BICOLORED: A crystal that displays two distinctly different colors with a change in viewing angle.

BITUMINOUS: Rock that contains such organic compounds as asphalt, tar, or petroleum.

BITUMINOUS COAL: A type of coal, called “soft coal,” containing 46 to 85 percent carbon that formed from medium-grade metamorphism of lignite.

BLADED: Having an elongated, flat crystal habit.

BORATE: A mineral consisting of one or more metals in combination with the borate radical (BO3)3- or similar boron-based radicals. Example: Probertite [basic hydrous sodium calcium borate, NaCaB5O7(OH)4·3H2O].

BOTRYOIDAL: A crystal habit, characterized by radiating crystals arranged in aggregates, that produces a smooth surface covered with spherical bulges that resemble bunches of grapes (from the Greek botrys, “bunch of grapes”).

BRILLIANCE: The degree of liveliness or “sparkle” in a gem that is determined mainly by cut and index of refraction.

BRITTLE: A mineral that is easily reduced to a powder by cutting or hammering.


CABOCHON: A gem cut in convex form and highly polished, but not faceted; a cut suited for translucent or opaque gems.

CALCAREOUS: Containing appreciable amounts of calcite [calcium carbonate, CaCO3].

CALDERA: In geology, a large, roughly circular, and frequently mineralized crustal subsidence formed by a collapsed volcanic system.

CARAT: The basic weight unit for gems; one carat equals one-fifth of a gram.

CARBONACEOUS: A rock consisting largely of organic carbon derived from plant or animal remains.

CARBONATE: A mineral containing one or more metals in combination with the carbonate radical (CO3)2-. Example: Smithsonite [zinc carbonate, ZnCO3].

CARBONATITE: A body of igneous rock consisting of more than 50 percent carbonate minerals.

CATION: An atom that has lost one or more electrons; a positively charged ion.

CATIONIC CHARGE: The total positive charge of a cation or a cationic radical. Example: The +2 charge of the lead cation Pb2+.

CHATOYANCY: The reflection of light from a translucent crystal or gemstone that resembles the slit in a cat’s eye and is caused the parallel arrangement of tiny, needle-like inclusions; also known as “cat’s-eye effect.”

CHEMICAL COMPOUND: A bonded combination of the ions of two or more elements.

CHEMICAL FORMULA: A conventional expression using chemical symbols to state the composition (and sometimes the structure) of a mineral in terms of the number of ions of each element present in that mineral’s molecule.

CHEMICAL SEDIMENTARY ROCK: A sedimentary rock formed by chemical processes such as precipitation. Example: Gypsum [hydrous calcium sulfate, CaSO4·2H2O].

CHROMATE: A mineral consisting of one or more metals and the chromate radical (CrO4)2-. Example: Crocoite [lead chromate, PbCrO4].

CHROMOPHORE: A color-causing agent; any element which in trace amounts can impart color to a mineral.

CLAY: A soft sediment or deposit that is plastic when wet and that consists largely of very fine, mica-like particles, usually of hydrous or basic aluminum silicates. Example: Kaolinite [basic aluminum silicate, Al2Si2O5(OH)4].

CLEAVAGE: The three-directional separation planes of minerals that are determined by internal atomic structure.

COLOR CENTER: An imperfection or distortion in a crystal lattice that traps electrons, alters the light-absorption characteristics of the lattice, and is an important color-causing mechanism in certain crystals.

COMPOSITE SPECIMEN: A specimen containing more or less equal amounts of two or more different mineral species.

CONCENTRATE: In mining, the valuable portion of ore that has been separated from gangue, or waste, material.

CONCHOIDAL: A fracture surface in a mineral marked by smooth, curved, shell-like patterns.

CONCRETION: A discrete, rounded, nodular mass of rock within sedimentary strata that consists of mineral particles cemented together.

CONTACT METAMORPHISM: A type of extreme metamorphism that takes place in the contact zone between an intruding body of magma and the existing rock.

CONTINENTAL DRIFT: The slow movement of large sections of the Earth’s crust that accounts for the present positions of the continents and seas; also known as “tectonic-plate movement.”

CORE DRILLING: The collecting of rock and mineral samples by drilling with a hollow bit and steel that yields core-shaped samples; the conclusive method of “proving” the existence of, and determining the richness and extent of, subterranean mineralization.

COVALENT BOND: The force that holds together two nonmetallic ions and which results from the mutual sharing of electrons between those ions. Example: The force that joins the oxygen and silicon ions in the silica tetrahedron (SiO4)2-.

CRUST: The uppermost layer of the Earth that lies atop the upper mantle.

CRYSTAL: A solid mass of a mineral having a regular geometric shape and an orderly internal structure; a crystal is bounded by smooth, flat surfaces called faces that are separated by well- defined edges.

CRYSTALLOGRAPHY: The study of the forms and structures of crystals, and of the mathematical relationships of crystal faces and axes.

CRYSTAL SYMMETRY: The repetitive patterns of crystal faces that are determined by a crystal’s internal, atomic structure.

CRYSTAL SYSTEM: A group into which minerals are categorized according to the symmetry and internal axial relationships of their crystals; the six crystal systems are isometric (cubic), hexagonal (including the trigonal subsystem), tetragonal, orthorhombic, monoclinic, and triclinic.

CUBE: A crystal shape with six, equal, square sides.

CUBIC CRYSTAL SYSTEM: A group of crystal forms that is characterized by three axes of symmetry, all of equal length and at right angles to each other; also called isometric. One of the six crystal systems.

CUPRIC: Containing divalent copper (Cu2+). CUPROUS: Containing monovalent copper (Cu1+).

CUT: In gemology, the proportions of the style and shape of a faceted gem as determined by the angles and symmetry of its facets.

DANA CLASSIFICATION NUMBER: The identifying number assigned to each mineral species under the Dana Mineral Classification System.

DANA MINERAL CLASSIFICATION SYSTEM: A widely used system of classifying minerals by chemistry and structure that was devised by American mineralogist James Dwight Dana.

DENDRITIC: Containing dark inclusions that have a tree-like or fern-like appearance; arborescent. Example: “Scenic” or “moss” agate.

DENSITY: A comparison of a mineral’s weight per unit volume that is expressed as specific gravity.

DEPOSIT: A natural concentration of a mineral or group of minerals that has or may have economic value.

DIMENSIONAL STONE: A rock that is quarried in blocks for architectural or construction purposes. Examples: Granite, marble.

DISPERSION: The ability of certain gemstones or gems to refract white light and separate it into its spectral components; dispersion produces the distinctive “fire” of diamond and zircon gems.

DODECAHEDRON: A crystal shape with 12 more-or-less-equal sides.

DRIFT: In mining, a horizontal passage in an underground mine.

DRUSY: A dense coating of small crystals with projecting terminations that covers the walls of a vein or vug.


EARTHY: A fracture surface on a mineral similar to that of broken clay or chalk.

EFFERVESCENCE: The rapid release of carbon-dioxide bubbles produced by the attack of an acid on a carbonate mineral. Example: The vigorous bubbling that accompanies the reaction of hydrochloric acid and calcite.

EFFLORESCENT: A mineral habit that appears as a thin crust on the surface of a rock or other mineral and that forms from evaporation of mineral-rich water.

ELASTIC: A mineral that will bend and return to its original form after stress is released.

ELEMENT: A fundamental material consisting of its own unique atoms. Examples: Iron, oxygen.

ELUVIAL: A secondary deposit formed by in-situ weathering of a primary deposit that is sometimes followed by limited gravitational movement.

ENVIRONMENT: In mineralogy, the specific chemical, physical, and geologic conditions under which a mineral forms.

EPITHERMAL VEIN: A hydrothermal, mineralized vein that forms in shallow conditions at low pressure and at low temperatures between 212° F. (100° C.) and 390° F. (200° C.).

EQUANT: An equidimensional crystal in which length, width, and thickness are approximately equal.

EROSION: The weathering, deterioration, and eventual movement of surface rocks and minerals that cause a general reduction of the Earth’s surface.

EUHDRAL: Well-developed crystal form.

EVAPORITE: A mineral formed at or near the surface through precipitation caused by the evaporation of aqueous solutions. Example: Halite [sodium chloride, NaCl].

EXTRUSIVE ROCK: An igneous rock that forms on the surface from the solidification of lava.


FACE: In crystallography, a smooth, flat, natural surface on a crystal.

FACET: In gemology, a smooth, flat, artificially created surface on a gem.

FADEN: A type of transparent quartz crystal characterized by a milky-white, fibrous, or feathery zone at its center that was caused by hydrothermal fluids that entered the crystal through microcracks and became trapped.

FAULT: In geology, a fracture where once-continuous rocks have been displaced along horizontal or vertical planes, with displacement varying from inches to thousands of feet.

FERRIC: Containing trivalent iron (Fe3+).

FERROUS: Containing divalent iron (Fe2+).

FERRUGINOUS: Containing iron.

FIBROUS: A crystal habit characterized by aggregates of thread-like fibers.

FLEXIBLE: A mineral that can be bent, but which will not return to its original form after stress is released.

FLOATER: A distorted, “free-form” crystal that developed within hydrothermal fluids while not attached to a base or matrix.

FLUORESCENCE: A type of luminescence caused by the absorption of electromagnetic energy and the immediate release of this energy in a wavelength of visible light. Example: The glow of colors emitted by certain minerals under ultraviolet light.

FLUX: A material employed in metal-smelting processes to reduce melting temperature and to chemically remove impurities. Example: Fluorite [calcium fluoride, CaF2].

FOSSIL: A remnant, impression, or trace of an organism of past geologic ages that has been preserved in the Earth’s crust.

FRACTURE: The irregular breakage characteristics that a mineral exhibits other than along cleavage planes, and that are described by such terms as uneven, conchoidal, hackly, splintery, and earthy.

FULGARITE: A form of natural, silica glass that forms when sand is melted by the impact of a lightning strike.

FUMAROLE: A vent in a volcanic area through which hot smoke and gases escape and around which some minerals, such as sulfur, may condense.


GANGUE: Minerals or rocks that are mined along with ore, but have no economic value and are discarded as waste.

GEM: An object fashioned from a gemstone. GEMOLOGY: The scientific study of gemstones and gems.

GEMSTONE: A mineral with ornamental value that can be fashioned into gems or small decorative objects.

GEODE: A hollow, generally spherical-shaped cavity in solid rock that is partially filled with secondary crystals or minerals.

GEOLOGIC TIME: The immense period of time that encompasses the natural history of the Earth, is measured in thousands to billions of years, and is divided into eons, eras, periods, and epochs.

GEOLOGY: The scientific study of the history and formation of the crust of the Earth as recorded in rocks.

GEOPHYSICAL EXPLORATION: The term employed for modern-day prospecting that involves the collection and interpretation of vast amounts of physical and chemical data.

GEOPHYSICAL RADIATION: The natural emission of alpha, beta, and gamma radiation from radioactive minerals that contain uranium, radium, or thorium. Geophysical radiation can sometimes induce colors in certain nonradioactive minerals.

GOSSAN: A near-surface, rusty or reddish-colored, mineralized deposit rich in hydrous iron oxides that overlies a sulfide deposit and forms through the oxidation of sulfide minerals.

GRABEN: In geology, a block of the Earth’s crust that has dropped between two fault systems. A rift is a graben that extends a long distance and appears as a surface subsidence.

GRAIN: A mineral or rock particle that is usually less than a millimeter or two in size and lacks well-developed crystal faces.

GRANULAR: Consisting of an aggregate of visible, similar-sized mineral grains or tiny crystals. GREASY: Having an oily luster.
GROUNDWATER: Water existing within rock below the surface.

GROUP (MINERAL): An informal classification of minerals that share certain chemical and physical properties and that are often cited by their group name. Example: The tourmaline-group minerals that include elbaite, dravite, etc.

GWINDEL: A quartz crystal with a helicoidal or spiral structure that formed from the distorting effects of positive and negative electrical charges on different faces during crystal development.


HABIT: The arrangement and proportions of the faces of a crystal, and thus the general shape of the crystal which may or may not resemble the ideal shape of the particular crystal system. Crystal habits are described by such terms as prismatic, acicular, granular, bladed, etc.

HACKLY: A fracture surface characterized by sharp, jagged edges similar to the surface of shredded metal.

HALIDE: A mineral consisting of a metal and a halogen. Example: Halite [sodium chloride, NaCl].

HARDNESS: The ability to resist abrasion.

HEXAGONAL CRYSTAL SYSTEM: A group of crystal forms that is characterized by four axes of symmetry, three of equal length and lying in a common plane at angles of 120°; the fourth axis is longer or shorter and is at right angles to the other three. One of the six crystal systems. (The former trigonal system is now included as a subdivision of the hexagonal system.)

HOPPER: A crystal with protruding edges and recessed faces that often shows repetitive, internal, parallel edges. Hopper crystals, also known as skeletal and window crystals, form when very rapid crystal growth provides insufficient time for normal development of the faces.

HYDROGEN BOND: The attractive force that attaches water molecules to hydrous minerals and that results from the dipolar property of the water molecule.

HYDROTHERMAL: A type of mineral emplacement in which precipitation of minerals from superheated, aqueous solutions creates vein- or replacement-type mineral deposits.

HYDROUS: Containing attached molecules of water; also known as hydrated. Example: Gypsum [hydrous calcium sulfate, CaSO4·2H2O].

HYDROXIDE: A mineral containing a metal and the hydroxyl radical or ion (OH)1-. Example: Brucite [magnesium hydroxide, Mg(OH)2].

HYDROXYL ION: The negatively charged combination of an oxygen ion with a hydrogen ion to form the radical (OH)1- which is an essential component of all basic minerals.

HYPOTHERMAL VEIN: A mineralized, hydrothermal vein that forms at great depth in conditions of high pressure and high temperatures between 575° F. (300° C.) and 930° F. (575° C.)


IDIOCHROMATIC: Literally “self-colored”; minerals in which color is caused by essential elemental components or the nature of the crystal lattice.

IGNEOUS ROCK: A crystalline rock that formed directly from the solidification of magma; may be intrusive or extrusive.

INCLUSION: A foreign mineral crystal or a liquid- or gas-filled cavity that is enclosed within a crystal.

INDEX OF REFRACTION: A measurement of a transparent crystal’s ability to refract or bend light; specifically, the proportional relationship between the velocity of light in air and its velocity within a crystal. High indices of refraction create brilliance in faceted gems.

INDICATOR MINERAL: In mining, a readily identifiable mineral, the presence of which can infer the nearby existence of other valuable ore minerals. Example: Bright-red realgar [arsenic sulfide, As4S4] can be an indicator of silver, copper, or lead minerals.

IN SITU: In its original position or place; a mineral deposit that has not been moved by mechanical weathering or alluvial transportation.

INTERNATIONAL MINERALOGICAL ASSOCIATION (IMA): The international organization of mineralogists founded in 1958 that approves, names, and lists mineral species.

INTRUSIVE ROCK: An igneous rock that formed underground from the solidification of magma within layers or formations of preexisting rocks.

IONIC BOND: The force resulting from the attraction of opposite electrical charges that holds together a distinctly metallic element and a distinctly nonmetallic element. Example: The force that bonds the mineral halite [sodium chloride, NaCl].

IRRADIATION: In gemology, the bombardment of a gemstone or gem with gamma radiation to alter is color.

IRIDESCENCE: A lustrous, rainbow-like play of colors caused by optical interference of light waves that changes with the angle of viewing.

ISOMETRIC (CUBIC) CRYSTAL SYSTEM: A group of crystal forms that is characterized by three axes of symmetry, all of equal length and at right angles to each other; also called cubic. One of the six crystal systems.


KARAT: A term describing the fineness of gold that is equal to a 1/24th part of pure gold in an alloy. Example: “24-karat” gold refers to pure gold.


LABRADORESCENCE: A display of dark, shimmering, metallic-like colors, usually green, blue, and gold, that is reflected from a translucent mineral. Example: The shimmering color effect displayed by the labradorite variety of certain plagioclase feldspar-group minerals.

LAMELLAR: A crystal habit in which one direction is substantially shorter than the other two.

LATTICE: The arrangement of atoms or molecules with a crystal that determines structure, shape, certain physical properties, and sometimes color.

LAVA: Magma that has been extruded onto the surface through volcanic action. LENTICULAR: Lens-shaped.

LEVEL: In mining, a network of passages or drifts on the same horizontal plane within an underground mine.

LIGNITE: A type of coal, called “brown coal,” that contains 25 to 35 percent carbon and forms from the burial and compression of peat; lignite is the lowest form of coal.

LITHIFICATION: The chemical and physical processes that consolidate loose sediments into solid rock.

LOCALITY: The place where a specific mineral specimen was collected; also, a notable source of specimens of a particular mineral.

LUMINESCENCE: The visible light emitted by some minerals while or after absorbing electrical, mechanical, thermal, or electromagnetic energy. Forms of luminescence include fluorescence, phosphorescence, thermoluminescence, and triboluminescence.

LUSTER: The manner in which light is reflected from the surface of a mineral. Terms used to describe luster are earthy, metallic, vitreous, pearly, resinous, silky, greasy, and adamantine.


MACROCYSTALLINE: Crystals large enough to be seen with the unaided eye.

MAFIC: A dark-colored, igneous or metamorphic rock that is rich in iron and magnesium minerals.

MAGMA: Molten rock material from the mantle of the Earth that solidifies to form igneous rocks at or below the Earth’s surface.

MAGNETISM: The force that enables some materials to attract or repel similar materials and that is caused by a magnetic field generated by a flow of electrons.

MALLEABLE: A mineral that can be deformed by hammering into thin sheets. Example: Gold [native metal, Au].

MANTLE: The subterranean portion of the Earth that lies between the crust and the central core.

MASSIVE: Consisting of interlocking mineral grains or crystals that are too small to be individually differentiated by the unaided eye.

MATRIX: The fine-grained material or groundmass that surrounds larger particles or crystals in igneous or sedimentary rocks; in mineral-collecting usage, the material in which fossils or primary crystals are embedded.

MESOTHERMAL VEIN: A mineralized vein that forms at moderate depth and pressure, and at moderate temperatures between 390° F. (200° C.) and 575° F. (300° C.).

METAL: An electropositive element that exhibits luster, malleability, conductivity, and ductility. METALLIC: Having a luster similar to that of polished metal.

METALLIC BOND: The force that holds together the ions of metallic elements and is caused by the mutual sharing of a pool of free-moving electrons.

METALLURGY: The study of the physical and chemical properties of metals, and the methods of extracting metals from ores.

METAMORPHIC ROCK: Rock that has been altered from a preexisting rock through heat, pressure, or chemical change, or a combination thereof.

METAMORPHISM: The process by which heat, pressure, and/or chemical changes alter existing rocks into other rocks exhibiting different textures and containing different minerals.

METAPHYSICAL: An aspect of the study of minerals and crystals that is based on both ancient lore and modern beliefs that transcend scientific study.

METASOMATISM: A radical change in chemical composition that occurs during metamorphism and results from the introduction of different elements and minerals from adjacent rocks.

METEORITE: The portion of a meteor that reaches the surface of the Earth without being vaporized and which can have stony, iron, or stony-iron composition.

MICACEOUS: Forming thin, flat, easily separated, mica-like sheets.

MICROCRYSTALLINE: Crystals that can be seen only with the aid of a microscope; also called cryptocrystalline. Example: The tiny, interlocking quartz crystals in chalcedony [silicon dioxide, SiO2].

MICROMOUNT: A small mineral specimen, usually containing distinct crystals, that requires magnification for study.

MILLING: In mining, the crushing, grinding, and concentration of ores that precede the smelting or refining process.

MINERAL: A naturally occurring, homogenous solid of inorganic origin with a definite chemical composition (within established limits) and an ordered crystalline structure.

MINERAL GROUP: An assemblage of three or more mineral species with similar chemical formulae and the same general crystal structure. Examples: The tourmaline-, garnet-, and zeolite-mineral groups.

MINERALIZATION: In mining, concentrated deposits of minerals that may be of economic importance.

MINERALOGY: The scientific study of the chemical and physical properties, structure, occurrence, distribution, identification, and classification of minerals.

MINERALOID: A mineral-like material that, for reasons of organic origin, lack of crystal structure, or indeterminate chemical composition, fails to meet the definition of a mineral. Examples: Obsidian, opal.

MOHS HARDNESS SCALE: The widely used scale of relative mineral hardness devised by Friedrich Mohs that assigns minerals hardness ratings from 1 (softest) to 10 (hardest).

MOLECULE: The fundamental unit of a chemical compound that consists of two or more atoms bonded together.

MOLYBDATE: A mineral consisting of one or more metals in combination with the molybdate radical (MoO4)2-. Example: Wulfenite [lead molybdate, PbMoO4].

MONOCLINIC CRYSTAL SYSTEM: A group of crystal forms that is characterized by three axes of unequal length, two lying in a common plane and the third inclined to that plane. One of the six crystal systems.

MULTIMETAL DEPOSIT: A mineralized deposit containing two or more economically valuable metals that is exploited by a multimetal mine. Example: Lead-silver-zinc mines.


NATIVE METAL: A metallic element that occurs in a free, uncombined form. Example: Copper [native metal, Cu].

NATIVE NONMETAL: A nonmetallic element that occurs in a free, uncombined form: Examples: Graphite, diamond (carbon, C).

NODULE: A small, rounded concretion consisting of mineral or rock particles that occurs in sedimentary formations.

NUGGET: A waterworn, irregular, mass of metal, such as gold, found in alluvial deposits.


OCTAHEDRON: A crystal shape with eight, more-or-less- equal sides.

OPALESCENCE: A pearly or milky-blue, internal glow that is caused by reflection of the shorter, mostly blue wavelengths of white light.

OPAQUE: Not transmitting light to any appreciable extent.
OPEN PIT: A surface excavation from which ore, as opposed to dimensional stone, is mined.

ORE: In mining, any rock or mineral containing a valuable constituent for which it can be mined, processed, and marketed profitably at the current time.

OROGENY: A mountain-building episode in which large sections of the Earth’s crust are uplifted. Example: The Laramide Orogeny that created North America’s modern Rocky Mountains.

ORTHORHOMBIC CRYSTAL SYSTEM: A group of crystal forms that is characterized by three axes of symmetry, each of unequal length and at right angles to one another. One of the six crystal systems.

OUTCROP: A mass of bedrock that is exposed at the surface.

OXIDATION: A chemical process in which contact with atmospheric oxygen and groundwater alters sulfide minerals into such oxygen-bearing minerals as oxides, carbonates, sulfates, arsenates, phosphates, etc.

OXIDATION STATE: The electrical charge, or valance, of an ion that indicates the number of electrons it will take or give up in a chemical reaction. Example: The oxidation state of the oxygen ion is -2.

OXIDE: A mineral consisting of a metal in combination with oxygen. Example: Corundum [aluminum oxide, Al2O3].

OXIDIZED ZONE: The upper portion of a sulfide-mineral body that has been altered by contact with atmospheric oxygen and groundwater.


PALEONTOLGY: The study of ancient life-forms through the interpretation of the fossil record.

PEARLY: Having a luster similar to the surface of a pearl.

PEAT: A partially carbonized material formed from the incomplete decomposition of buried plant remains; a precursor to coal.

PEGMATITE: An igneous rock of extremely coarse grain size that occurs as dikes or lenses within a larger, fine-grained rock mass and is often a source of unusual minerals and large crystals.

PETROLOGY: The study of the composition, genesis, occurrence, and classification of rocks.

PHANTOM: A feathery, internal form within a transparent crystal that replicates the external form and was caused by a disruption in growth when a secondary crystal grew over the original crystal in the same orientation.

PHENOCRYST: A prominent crystal surrounded by smaller grains in a porphyritic rock.

PHOSPHATE: A mineral with one or more metals in combination with the phosphate radical (PO4)3-. Example: Apatite-(CaF) [calcium fluorophosphate, Ca5(PO4)3F].

PHOSPHORESCENCE: The visible light emitted by some minerals after absorbing electrical, mechanical, or electromagnetic energy and after the source of the energy has been removed.

PIEZOELECTRICITY: The electrical current that is generated across the faces of certain mineral crystals when subjected to mechanical stress.

PIPE: A cylindrical, vertical, or nearly vertical mass of intruded, igneous rock.

PLACER: A concentrated deposit of heavy mineral particles that formed through the gravitational sorting action of moving water.

PLAYA: A closed, shallow basin in which water collects and evaporates.

PLEOCHROISM: The ability of a colored crystal to display another color or color intensity when the viewing angle is changed, and that results when light from varying directions is absorbed and reflected differently.

POLYMORPHIC: Two or more minerals with identical chemistries, but different crystal structures and thus classified as different species. Examples: Kyanite, andalusite, and sillimanite [aluminum silicate, Al2SiO5].

PORPHYRITIC ROCK: An igneous rock in which larger crystals are enclosed in a fine-grained groundmass.

PRECIPITATION: The process is which a chemical compound is separated from solution in the form of a solid.

PRIMARY MINERAL: In geology, a mineral occurring in its original emplacement that has not been chemically or physically altered; in mineral collecting, the dominant mineral in value and visual prominence within a composite specimen.

PRISMATIC: A crystal that is appreciably longer in one direction than in the other two directions.

PSEUDOMORPH: A mineral that has replaced a preexisting mineral of a different species and that has assumed that mineral’s external shape.

PYRAMIDAL: A crystal form in which three, four, six, eight, or twelve nonparallel faces meet at a common point.

PYROELECTRICITY: The electrical current that is generated across the faces of certain crystals when heated.


QUARRY: A surface excavation from which rock and dimensional stone, as opposed to ore, are extracted in intact blocks for architectural or construction purposes; also, a surface working from which such materials as limestone are extracted for processing.


RADIATING: Crystals growing outward from a central point.

RADICAL: A group of atoms of different elements that are bound together and act as an entity or as a singular ion in chemical reactions. Example: The sulfate radical (SO4)2-.

RADIOACTIVITY: Energy in the form of alpha, beta, and gamma radiation that is emitted by certain minerals with unstable nuclei, such as those containing the elements radium, thorium, and uranium.

REDUCTION: In metallurgy, a chemical reaction in which elemental metals are recovered from concentrated ores. Example: The reduction of lead from galena [lead sulfide, PbS] concentrates.

REGIONAL METAMORPHISM: A type of metamorphism in which huge masses of rock over broad areas are altered by prolonged and intense heat and pressure generated by deep burial.

RENIFORM: Literally “kidney-shaped”; a mineral with a rounded, kidney-shaped surface. RESINOUS: Having the luster of resin.

RIFT: In geology, a regional graben or subsidence between two geological faults that is topographically manifested as a long, broad valley or, when flooded, an elongated sea or gulf.

ROCK: A cohesive aggregate of two or more minerals of indefinite mineralogical composition. Example: Granite, an aggregate of quartz, feldspar-group, and mica-group minerals.

ROSETTE: An intergrowth of numerous platy crystals arranged like the petals of a rose. ROUGH: A gemstone in its natural state, before it is fashioned into a gem.


SCEPTER: A prismatic crystal that is largest in diameter at its termination, thus creating a shape that resembles a royal scepter.

SECONDARY MINERAL: In geology, a mineral formed by the alteration of a primary mineral; in mineral collecting, an associated mineral of lesser value and visual prominence than the primary mineral within a composite specimen.

SECTILE: A mineral that can be cut with a knife into thin shavings.

SEDIMENTARY ROCK: A layered, consolidated rock formed through the accumulation of particles of minerals, rocks, and organic matter that are cemented together. Example: Sandstone.

SELENIDE: A mineral consisting of two or more metals combined with the semimetallic element selenium. Example: Stilleite [zinc selenide, ZnSe].

SEMIMETAL: An element that exhibits certain properties of both metals and nonmetals; also known as a metalloid. Examples: Arsenic, antimony.

SHAFT: In mining, a vertical passage connecting the surface with underground workings of a mine. Internal shafts are vertical passages connecting the interior levels of a mine.

SILICA: Silicon dioxide.

SILICATE: A mineral consisting of one or more metals in combination with silicon and oxygen. Example: Zircon [zirconium silicate, ZrSiO4].

SILICIFICATION: The replacement of, or emplacement within, a mineral or organic material by silica.

SILKY: Having the luster of silk.

SKARN: A mineralized deposit that forms through processes of metamorphism and metasomatism after magma has intruded formations of limestone or dolomite rock.

SKELETAL: Hollow crystals that form by very rapid crystallization.

SMELTING: The extraction of metal from concentrated ores by pyrometallurgical processes that involve heat and chemical reactions.

SOLID-STATE SOLUTION: A compositional relationship that occurs when two elements substitute for each other in two minerals to produce a chemically graded series with identical structures. Example: The fayalite-fosterite (iron-magnesium silicate) series in which iron substitutes for magnesium and vice versa.

SPECIES: In mineralogy, a mineral with unique chemical and/or physical properties that differentiate it from all other minerals.

SPECIFIC GRAVITY: A mathematical expression of density; the relative weight of a mineral compared to that of an equal volume of water.

SPECIMEN: A sample of a mineral that exhibits all or most of the typical characteristics of that species, or that exhibits unusual characteristics; in mineral collecting, a fine specimen exhibits exceptionally well-developed characteristics of a particular species.

SPECIMEN MINING: The commercial collection of minerals to sell as specimens.

SPECTROGRAPHIC ANALYSIS: A qualitative, investigative technique in which mineral samples are energized and induced to emit characteristic patterns of electromagnetic radiation that are unique and identifiable for each specific element present.

SPLINTERY: A fracture surface in a mineral that is marked by elongated splinters or fibers.

STALACTITIC: A mineral deposit resembling an icicle that hangs from the roof of a cave or vug and forms from the drip and evaporation of mineral-rich water.

STALAGMITIC: A mineral deposit resembling an inverted icicle that forms on the floors of caves or vugs from the drip and evaporation of mineral-rich water.

STOPE: In mining, an underground working in which ore is removed in successive layers.

STREAK: The often-diagnostic color of a mineral when it is ground into a fine powder; usually produced by scraping a mineral firmly across a tile of unglazed, white porcelain called a “streak plate.”

STRIATIONS: The minute, parallel grooves or lines that appear on some crystal faces and that are a diagnostic feature of certain minerals.

SULFATE: A mineral consisting of one or more metals in combination with the sulfate radical (SO4)2-. Example: Barite [barium sulfate, BaSO4].

SULFIDE: A mineral consisting of one or more metals or semimetals in combination with sulfur. Example: Pyrite [iron disulfide, FeS2].

SULFOSALT: A mineral consisting of one or more metals and one or more semimetals in combination with sulfur. Example: Pyrargyrite [silver antimony sulfide, Ag3SbS3].


TABULAR: A crystal habit characterized by thick or thin plates.

TAILINGS: The fine-grained remnants of ore left behind after the valuable minerals or metals have been extracted.

TARNISH: A thin, sometimes iridescent film of chemically altered material on the surface of a mineral that usually forms through the combination of a metal with oxygen or sulfur. Example: The blue, iridescent film on the surface of bornite [copper iron sulfide, Cu5FeS4].

TEKTITE: A natural glass of variable composition that is sometimes used as a gemstone and that formed from meteoritic impact on high-silica rock and sand. Example: Moldavite.

TELLURIDE: A mineral consisting of one or more metals in combination with the semimetal tellurium. Example: Calaverite [gold telluride, AuTe2].

TENACITY: The manner in which a mineral reacts to the stress of crushing or breaking, and which is described by such terms as brittle, sectile, malleable, flexible, and elastic.

TERMINATION: The end of a crystal opposite the base that forms a point, edge, or flat surface.

TETRAGONAL CRYSTAL SYSTEM: A group of crystal forms that is characterized by three axes of symmetry, two of equal length and lying in a common plane at 90°, and the third being longer or shorter and at a right angle to the other two. One of the six crystal systems.

THERMOLUMINESCENCE: The visible light emitted by some minerals after they are heated.

TRANSLUCENT: Transmitting some light, but diffusing the light so that objects beyond cannot be clearly seen.

TRANSPARENCY: The degree to which light passes through a mineral; described as transparent, translucent, or opaque.

TRANSPARENT: Transmitting virtually all light, enabling objects beyond to be clearly seen.

TREATED GEMS: Gems in which color, clarity, finish, or strength have been enhanced, usually by such techniques as heating, irradiation, oiling, dying, lacquering, or sealing cracks with epoxy compounds.

TRIBOLUMINESCENCE: The visible light emitted by some minerals when placed under mechanical stress; also known as mechanoluminescence.

TRICLINIC CRYSTAL SYSTEM: A group of crystal forms that is characterized by three axes of symmetry, all of different lengths and none perpendicular to the others. One of the six crystal systems.

TRIGONAL CRYSTAL SUBSYSTEM: see “Hexagonal Crystal System.”

TUNGSTATE: A mineral consisting of one or more metals in combination with the tungstate radical (WO4)2-. Example: Hübnerite [manganese tungstate, MnWO4].

TWIN: Two or more single crystals that have grown together along a common plane and in a definite, systematic arrangement.

TYPE LOCALITY: The source of the specimens that provided the data used to define a particular mineral as a new mineral species.

TYPE SPECIMEN: The specific mineral sample that was collected from a type locality and studied to define a new mineral species.


ULTRAVIOLET LIGHT: Electromagnetic radiation with a wavelength of 4 to 400 nanometers that is just beyond violet in the visible spectrum; also called “black light.” Ultraviolet light is commonly used to energize certain minerals to display fluorescence.

UNEVEN: An irregular fracture surface in a mineral.

UNIT CELL: The smallest part of a mineral that still retains the unique atomic structure and physical properties of the mineral.

UPLIFT: A section of the Earth’s crust, such as nonvolcanic mountains and plateaus, that tectonic stresses have thrust upward


VANADATE: A mineral consisting of one of more metals in combination with the vanadate radical (VO4)3-. Example: Vanadinite [lead chlorovanadate, Pb5(VO4)3Cl].

VAN DER WAALS FORCES: A very weak molecular bond that is formed by the attraction of the positively charged, electron-deficient part of an electrically neutral molecule to the negatively charged, electron-rich part of an electrically neutral molecule.

VARIETY: In mineralogy, a subclassification of a species in which a unique color or other property establishes a distinctive identity. Example: The amethyst variety of quartz [silicon dioxide, SiO2].

VEIN: A sheet-like or tabular, mineralized mass that occupies a fracture or fracture system within a host rock.

VESICLE: A small, barren cavity in a volcanic rock that is formed by gas bubbles within solidifying lava.

VITREOUS: Having a glassy luster.

VOLCANO: A vent in the Earth’s surface through which magma and related ash and gases extrude onto the surface or are ejected into the atmosphere.

VUG: A general term for a cavity in a rock or vein.


WATER OF HYDRATION: Water molecules that are attached to hydrous minerals.

WEATHERING: The processes of chemical alteration and mechanical deterioration of rocks and minerals at or very near to the surface that is caused by exposure to air, water, acids, and mechanical stresses.

WHITE LIGHT: Light that appears colorless but which consists of all spectral colors.


X-RAY DIFFRACTION: An analytical method that employs X rays that are passed through a crystal in which unique electron densities diffract the X-rays into distinctive “scatter patterns” that represent the crystal’s atomic structure.


ZEOLITE: One of a group of hydrous aluminosilicate minerals that have the unique ability to dehydrate or rehydrate without altering their crystal structures. Example: Thomsonite-(Ca) [Ca2Na(Al5Si5O20)·6H2O].

Steve Voynick (C) copyright Celestial Earth Minerals