A Glossary of Rock and Mineral Terminology

The glossary is arranged alphabetically

A

Term	Definition
acicular:	A mineral consisting of fine needle-like crystals.
adamantine:	A brilliant luster like that of a diamond.
aggregate:	a. A mass of rock particles, grains of minerals, or both. b. Irregular mass of crystals. c. Sand, gravel, crushed stone or rock that forms the major part of concrete.
alkaline:	Containing sodium and/or potassium in excess of the amount needed to form feldspar with the available silica. An alkaline rock, for example, contains more than average amounts of potassium- and sodium-bearing minerals.
amorphous:	Without form; applied to rocks and minerals which have no definite external crystalline structure.
amphibole:	A mineral group that consists of common, dark-colored, rock-forming silicate minerals. The most common minerals are hornblende, tremolite, and actinolite.
andesite:	A volcanic rock composed essentially of the mineral andesine and one or more of the mafic minerals.
anhedral:	Refers to a crystal with no well-formed external faces.
arborescent:	Minerals having a treelike form, with branches similar to the way a tree forms.
asbestos:	A group of naturally fibrous silicate minerals. The most common asbestos type is chrysotile.

B

Term	Definition
bed:	A rock mass of large horizontal extent bounded by physically different material.
bituminous:	a. Containing much organic, or at least carbonaceous matter, mostly in the form of the tarry hydrocarbons which are usually described as bitumen. b. Pertaining to bituminous coal.
bladed:	Elongated and flattened; descriptive of some minerals.
borates:	Compounds are formed when metallic elements combine with the borate radical. Minerals include ulexite, colemanite, and howlite.
boron mineral:	Many known minerals contain boron, but only a few are commercially valuable as a source of boron. The principal boron minerals are borax, kernite, colemanite, and ulexite.
botryoidal:	A mineral surface that resembles a cluster of grapes: describes certain minerals such as hematite and malachite.
brittle:	Breaks or powders easily. A type of tenacity.

C

Term	Definition
cabochan:	A style of cutting in which the top of the gemstone forms a curved convex surface. The base may be convex, concave, or flat.
carbonaceous:	a. Referring to a rock or sediment that is rich in carbon. b. Referring to a sediment containing organic matter.
carbonates:	Compounds in which one or more metallic or semimetallic elements combine with the carbonate radical. Calcite is the most common carbonate.
chalky:	Having the color, luster, or general appearance of chalk.
chemical formula:	The standard way of stating the chemical composition of a mineral in terms of the number of atoms of each element contained in that mineral.
clastic:	Consisting of fragments of minerals, rocks, or organic structures that have been moved individually from their places of origin. Synonym detrital.
cleavable:	Easily split into smaller fragments. See cleavage.
cleavage:	The property of a mineral breaking along its crystallographic planes.
color:	One of the most important properties used in determining minerals. Some minerals always show the same color, while others come in many different ones. The color of metallic (or metal-bearing) minerals is fairly constant; whereas that of non-metallic minerals is generally less so owing to the pigmentation effect of impurities. The color of a massive mineral is sometimes different from the color of its powder or streak.
columnar:	A mineral with a structure obscurely resembling prisms, for example, hornblende.
compact:	Dense crystalline texture requiring magnification to distinguish individual crystals or particles.
conchoidal:	Shell-shaped; the more compact rocks, such as flint, which break with concave and convex surfaces, are said to have a conchoidal fracture.
concretion:	A hard, compact mineral-mass of mineral matter that forms usually in sedimentary rock around a center such as bone, shell, leaf, or fossil.
conglomerate:	Refers to a cemented clastic rock containing rounded fragments of pebble size gravel; the consolidated equivalent of gravel. The composition of rock or mineral fragments may vary widely in size, but are usually rounded and smoothed from transportation by water or wave action.
coralloidal:	Having the form or appearance of coral.
cryptocrystalline:	Crystalline, but so fine-grained that the individual components cannot be seen under an ordinary microscope.
crystal:	The geometrical, faceted shape assumed naturally by minerals as they solidify, provided that nothing impedes their growth. A crystal is characterized first by its definite internal, molecular structure and second by its external form.
crystal class:	One of the 32 possible crystallographic combinations or groups of symmetry operations that leave one point, or origin, fixed. Crystal classes are divided among the six crystal systems and deal with outward symmetry.
crystal face:	The flat exterior surface of a crystal.
crystal form:	The form or shape in which crystals occur, such as the cube, the octahedron, and others.
crystal habit:	Crystal habit is a typical crystal form or a combination of forms or other shapes. It is the general shape of crystals that play an important role in identifying minerals.
crystal system:	Minerals are classified in one or more of the six designated crystal systems, according to the geometrical shapes in which minerals crystallize. These are isometric, tetragonal, hexagonal, orthorhombic, monoclinic and triclinic.
crystalline:	Made of crystal or resembling crystal. Crystalline rock is composed of crystals or fragments of crystals. Opposite of amorphous.
crystallography:	The study of crystals, including their growth, structure, physical properties, and classification by form.
cube:	A crystal form of six equivalent and mutually perpendicular faces.
cubic:	Having the form of a cube, as a cubic crystal.
cubo-octahedron:	A crystal form which has faces of both the cube and the dodecahedron.

D

Term	Definition
dendrite:	A branching figure resembling a tree produced on or in a mineral or stone by an oxide of manganese or other foreign mineral (as in the moss agate); the mineral or stone so marked.
dendritic:	Branch- and moss-like shapes or markings. For example, some crystallized native gold.
dense:	a. Compact, fine-grained, lacking pore space. b. Rock or mineral with a high specific gravity.
deposition:	Earth material of any type that has accumulated by some natural process and is large enough to invite exploration, such as a mineral or ore.
detrital:	See clastic.
dike:	A tabular body of igneous rock injected into a fissure when molten and cuts across the structure of the adjacent rocks.
dipyramidal:	A crystal form where two identical pyramids are joined base to base.
disseminated:	Fragments of minerals dispersed in a rock.
distorted crystal:	A crystal whose faces have developed unequally, some being larger than others. Some distorted crystal forms are drawn out or shortened, but the angle between the faces remains the same.
dodecahedral:	A crystal form with 12 faces. Each face (either pentagonal or rhombic) is parallel to one crystallographic axis and intersects the other two at equal distances.
dodecahedron:	An isometric form composed of 12 equal faces that are either pentagonal or rhombic.
druse:	A cavity in a rock or vein with walls encrusted with small projecting crystals.
ductile:	a. The capacity of a metal to elongate without cracking or breaking, when pulled from the ends. b. Capable of being permanently drawn out without breaking.
dull:	Those minerals in which there is a total absence of luster, as chalk or kaolin.

E

Term	Definition
earthy:	Rough to the touch; dull and lusterless.
effervesce:	To bubble or hiss, as in carbonated water.
elastic:	Able to return to original shape after bending.
element:	A substance composed of atoms bearing an identical number of protons in each nucleus. An element cannot be decomposed into other substances (except by radioactive decay or bombardment with high-speed particles).
equant:	Applied to crystals having the same, or nearly the same, diameter or the same dimensions in all directions.
evaporation:	The change by which a substance is converted from a solid or liquid state into a vapor state.
evaporites:	a. One of the salts that results from the evaporation of ocean water or of saline lakes. b. Rocks such as rock salt and potash salts formed by evaporation of lakes or seas.

F

Term	Definition
face:	See crystal face.
feldspar:	A mineral group that consists of abundant, light-colored, rock-forming silicate minerals. The most common minerals are microcline, orthoclase, and plagioclase.
ferromagnesian:	Containing iron and magnesium. Applied to certain dark silicate minerals, especially amphibole, pyroxene, biotite, and olivine.
fiber:	The smallest single strands of asbestos or other fibrous materials.
fibrous:	a. Applied to minerals that occur in fibers, such as asbestos. b. Consisting of fine threadlike strands; for example, satin spar.
filiform:	A mineral occurring as thin threads, often twisted like the strands of a rope; for example, native copper.
fine:	Very small in size, weight, or bulk
flexible:	Bends without breaking and will not return to its original shape.
fluorescence:	The emission of visible light by a substance exposed to ultraviolet (UV) light. Some materials, when exposed to invisible UV radiation, emit visible light and seem to "glow-in-the-dark."
foliated:	Thin, leaf-like layers of intergrowth, such as micaceous or schistose rocks.
fracture:	a. The way in which a mineral breaks, other than its cleavage. b. A crack, joint, or break in rocks.

G

Term	Definition
geode	A hollow nodule or concretion, the cavity of which is commonly lined with crystals.
glassy:	Applied to igneous rocks that have no crystals, only supercooled magma. For example obsidian.
globular:	Smooth rounded surface of crystalline intergrowth.
gossan:	An iron-bearing weathered product overlying a sulfide deposit. It is formed by the oxidation of sulfides and the leaching-out of the sulfur and most metals, leaving hydrated iron oxides and rarely sulfates.
grain:	A mineral or rock particle, less than a few millimeters in diameter, and generally lacking well-developed crystal faces, for example a sand grain. Used to describe sedimentary particles of all sizes from clay to boulders, for example fine-grained and coarse-grained.
granular:	Consisting of grains of approximately equal size and ranging from two to ten millimeters.
greasy:	Applied to the luster of minerals; minerals that are oily to the touch or sight. For example talc.

H

Term	Definition
habit:	The characteristic or typical crystal form, combination of forms, or other shape of a mineral, including irregularities.
halides:	Compounds in which metallic elements combine with halagens (the elements, bromine, chlorine, fluorine, and iodine). Minerals include halite and fluorite.
hardness:	Hardness is the resistance of a mineral to abrasion or scratching.
hexagonal:	One of the six crystal systems. Hexagonal crystals have four axes of symmetry; three equal horizontal axes intersect at 120 degree angles, the fourth can be either longer or shorter and perpendicular to the plane of the other three.
hopper-shaped:	Crystal faces with depressed centers because growth has been quicker along their edges than at the center.
hydrocarbons:	Organic substances that are composed only of hydrogen and carbon.
hydrothermal veins:	Veins formed at relatively high temperatures (300 degrees - 500 degrees C) and at relatively great depth.
hydroxides:	Compounds of metallic elements combined with water and hydroxyl.

I

Term	Definition
igneous:	Rock formed by the solidification of molten rock (magma).
inlier:	An area or group of rocks surrounded by rocks of younger age.
intergrowth:	An interlocking of two or more minerals that resulted from their simultaneous crystallization.
iridescent:	Rainbow-like colors on the interior or surface of a mineral.
isometric:	One of the six crystal systems. Isometric crystals have three axes of equal length, all at right angles to one another.

L

Term	Definition
lamellar:	Composed of thin layers, plates, or scales; disposed in layers like the leaves of a book.
lathanides:	Rare earth elements from atomic numbers 58 to 71 inclusive. They have chemical properties similar to lanthanum, a silvery-white metal that can take a high polish and which is the most common and most basic of the rare earth metals.
lapidary:	A craftsman who cuts and polishes precious stones.
lignite:	A low-grade brownish black coal.
lode:	A deposit consisting of several veins spaced closely enough so that all of them, together with the intervening rock, can be mined as a unit.
lode gold:	Gold found in veins within quartz or other rock.
luster:	The way a mineral reflects light or shines. It is one of the means for distinguishing minerals.

M

Term	Definition
mafic:	Dark colored rock; usually applied to igneous and metamorphic rocks containing magnesium and iron rich minerals.
malleable:	Able to be hammered into thin sheets without breaking, as gold or silver. See tenacity.
massive:	a. A mineral deposit characterized by a great concentration of ore in one place, as opposed to a disseminated or vein deposit. b. A rock with a homogenous texture or fabric over a wide area, with an absence of layering, foliation, cleavage, or any similar directional structure. c. Many interlocking crystals, without faces, forming masses of considerable size.
metallic:	Minerals having the luster of a metal, for example gold and copper.
metallurgy:	a. The science and art of separating metals and metallic materials from raw ore by mechanical and chemical means. b. Study of the physical properties of metals as affected by composition, mechanical working and heat treatment.
metamorphic:	Refers to rock which has been altered by heat or intense pressure at a depth in the earth's crust causing new minerals and new structures in the rock to be formed.
mica:	A group of monoclinic minerals having similar chemical compositions. The micas are prominent rock-forming constituents of igneous and metamorphic rocks.
micaceous:	Composed of or resembling mica. Occurring in thin plates or scales like mica.
micromount:	A natural mineral specimen, preferably in distinct crystals, mounted, properly labeled, and requiring magnification for meaningful observation.
mineral:	A naturally occurring inorganic substance having an orderly internal structure and characteristic chemical composition, crystal form, and physical properties.
mineral classification:	The division of minerals into classes according to aspects of their chemical composition and atomic structure.
mineral name:	A distinct mineral name is given to each mineral species. Minerals have been named after people, places, properties, and similarities with other minerals. Today, all newly discovered minerals, and their intended new name, must be presented for evaluation to the Commission on New Mineral Names of the International Mineralogical Association (IMA) so that the name is sanctioned and standardized.
mineral species:	Any mineral that can be distinguished from all other minerals. It must differ from every other mineral species either in its chemical composition or atomic structure.
monoclinic:	One of the six crystal systems. Monoclinic crystals have three unequal axes, two of which are at right angles to one another and lie in a plane, the third is inclined to the plane of the other two.

N

Term	Definition
native	Occurring in nature, either pure or uncombined with other substances. Usually applied to metals, such as native mercury and native copper.
native element:	Uncombined elements that are classified into three groups: metals such as gold, semimetals such as antimony, and nonmetals such as sulfur.
nitrates:	Compounds in which one or more metallic elements combine with the nitrate radical, for example, nitratine.
nodule:	A small rounded lump of a mineral or aggregate, normally without internal structure, and having a contrasting composition from the enclosing sediment or rock in which it is embedded. For example, pyrite in a coal bed.
nugget:	A lump of native gold, silver, platinum, copper, etc.

O

Term	Definition
octahedral:	Referring to or resembling an octohedron, a crystal form consisting of eight triangular faces each having equal intercepts on all three crystallographic axes.
oily:	Poorly reflective luster, similar in appearance to oil.
oolitic:	Concentric globular deposits smaller than the size of a pea.
ore:	a. Rocks or minerals that can be mined for a profit. b. The mineral(s) thus extracted.
orthorhombic:	One of the six crystal systems. Orthorhombic crystals have three unequal axes, all lying at right angles with respect to one another.
outcrop:	The part of a rock formation that appears at the ground's surface.
overburden:	A material that readily yields oxygen or other oxidizing substances needed for an explosive reaction to take place.

P

Term	Definition
pearly:	Minerals with a luster like a pearl, such as talc.
pegmatite:	Coarse-grained igneous rock found usually as dikes associated with a large mass of finer grained plutonic rock.
pentagonal:	A polygon with five sides and five interior angles.
petrify:	To become stone. Organic substances such as shells, stones, and wood embedded in sediments become converted into stone by the gradual replacement of their tissues, particle by particle, with corresponding amounts of infiltrated mineral matter.
phosphates:	Compounds in which metallic elements combine with the phosphate radical. Minerals include lazulite, pyromorphite, and ludlamite.
phosphorescence:	Where a mineral continues to glow for an interval after the ultraviolet light source has been turned off. See fluorescence.
plagioclase:	A group of minerals containing a mixture of sodium and calcium feldspar.
platy:	Mineral habit with flat, thin crystals.
plumose:	Having a feathery appearance.
plutonic:	A general name pertaining to igneous rocks formed at great depths.
porcelaneous:	Dull, white luster resembling unglazed porcelain.
porous:	Contains voids, pores, cells, and other openings that may or may not interconnect.
prism:	A crystal form with three or more similar faces parallel to a single axis.
property:	One of the physical or chemical characteristics of a material.
pulverulent:	Easily reduced to powder.
pyramidal:	A crystal form which has three, four, six, eight, or twelve nonparallel faces that meet at a point.
pyritohedron:	An isometric closed crystal form of 12 faces, each having an irregular pentagon. It is named after pyrite, which characteristically has this crystal form.
pyroxene:	This mineral group consists of common, dark-colored, rock-forming silicate minerals. The most common minerals are diopside and augite.

R

Term	Definition
radiated:	Crystal aggregates that radiate from a center without producing star-like forms.
rare earths:	a. Oxides of a series of 15 metallic elements, from lanthanum (atomic number 57) to lutetium (71), and of two other elements, yttrium (39) and scandium (21). b. Rare earths are not especially rare in the earth's crust, but economic concentrations are. The rare earth metals resemble one another very closely in chemical and physical properties so it is difficult to separate them.
reniform:	A term meaning kidney-shaped, used to describe rounded mineral surfaces.
resinous:	Having the luster of resin.
reticulated:	A mineral with cross meshes, like a net.
rhombic:	Shaped liked a rhombus (an equilateral parallelogram).
rhombohedral:	Referring to a rhombohedron, which is a common crystal form in the hexagonal crystal system. Its 3-dimensional shape can be visualized as a cube stretched or compressed in the direction of two opposite corners.
rock:	An aggregate of one or more minerals, (such as granite, shale, marble) or a body of undifferentiated mineral matter, (such as obsidian) or a solid organic material, (such as coal). Rocks fall into three broadly defined groups: igneous, sedimentary, and metamorphic rocks. In some instances a single mineral forms a rock, as calcite, serpentine, and kaolin.
rock-forming minerals:	Mineral that is common and abundant in the earth's crust; one making up large masses of rock.
rosette:	Intergrowth of numerous platy crystals overlapping like the petals of a rose.

S

Term	Definition
scaly:	Consisting of scales or tabular crystals.
schistose:	Resembling schist (a crystalline rock that can be readily split or cleaved because of having a foliated or parallel structure).
sectile:	Capable of being cut with a knife into thin shavings.
sedimentary:	Pertaining to rocks formed by the sediment deposition (fragments of rocks, minerals, animal or plant material).
series:	Any number of rocks, minerals, or fossils having characteristics, such as growth patterns, succession, composition, or occurrence, that make it possible to arrange them in a natural sequence.
serpentine:	A group of common rock-forming minerals with a greasy or silky luster, a somewhat soapy feel, and a tough, conchoidal fracture. They are usually green, greenish yellow or greenish gray and often veined or spotted with green and white.
sheen:	The way a mineral reflects light can be affected by characteristics just below the mineral's surface: for example, the pearly sheen caused by partly-developed cleavage(s) parallel to the surface; the silky sheen from a fibrous growth structure or parallel hair-like inclusions.
silica:	A dioxide of silicon, which occurs in crystalline form as quartz, crystobalite, amorphous opal; an essential constituent of the silicate group of minerals.
silicates:	Compounds in which metallic elements combine with either single or linked Si-O tetrahedra. Stucturally, minerals in this class are subdivided according to their crystalline structure, into various groups (for example, ring silicates, single-chain silicates, framework silicates) rather than their chemical formula. Minerals include beryl, tourmaline, and epidote.
silky:	Having the luster of silk.
skeletal:	In crystallography, hollow or imperfectly developed crystals formed by rapid crystallization.
soluble:	Capable of being dissolved in a fluid.
spalling:	The chipping and upward and outward heaving of rock caused by release of pressure.
specific gravity:	The ratio of a specimen's weight compared with the weight of an equal volume of water. Therefore, if the weight of a mineral specimen is three times that of the volume of water it displaces, its specific gravity is three.
specimen:	A sample, as of a mineral, rock, ore, or fossil.
spherulitic:	A rounded mass of acicular crystals, radiating from a central point.
splintery:	The property of certain minerals or rocks to break or fracture into elongated fragments like splinters of wood.
spongy:	Applied to vesicular rock structures with thin partitions between the vesicles, thus resembling a sponge.
stalactitic:	Like a stalactite, a conical or cylindrical mineral deposit that hangs from the ceiling of a cave.
stellate:	An aggregate of crystals in starlike arrangement.
streak:	A mineral's powder color. It is most easily observed by rubbing the mineral on a piece of white unglazed porcelain (called a streak plate). Also, streak can be observed by fine crushing a specimen with a hammer.
subvitreous:	Luster not as highly reflective as adamantine, but more so than vitreous.
sulfates:	Compounds in which one or more metallic elements combine with the sulfate radical. Minerals include barite, gypsum, and linarite.
sulfides:	Compounds in which sulfur combines with metallic and semimetallic elements. Minerals include cinnabar, pyrite, and galena.
sulfosalts:	Compounds in which metallic elements combine with sulfur plus a semimetallic element, for example, stephanite.

T

Term	Definition
tabular:	Crystal habit with the appearance of a paper tablet.
tenacity:	The strength of a mineral; its resistance to breaking, crushing, bending, or tearing. Terms commonly used to describe the tenacity of a mineral are brittle, sectile, malleable, flexible, elastic, ductile, and tough.
tetragonal:	One of the six crystal systems. Tetragonal crystals have three axes at right angles of which only two lateral axes are equal.
tetrahexahedron:	A crystal form of the isometric system, bounded by twenty-four equal triangular faces, four to each face of the cube.
tough:	Strong and firm in texture but flexible and not brittle; capable of resisting great strain without coming apart.
triclinic:	One of the six crystal systems. Triclinic crystals share three axes, all having different lengths and perpendicular to the others.
trisoctahedron:	A form with 24 congruent triangular faces and having an octahedron base.
tufted:	Refers to a crystal aggregate in the form of fibrous crystal clumps.
tungstates:	Compounds in which metallic elements combine with the tungstate radical. Minerals include scheelite and wolframite.
twin crystal:	Two portions of a crystal having a definite crystallographic relationship.
twinning:	An intergrowth of two or more single crystals of the same substance.

V

Term	Definition
vanadates:	Compounds in which metallic elements combine with the vanadate radical, for example, carnotite.
variety:	A mineral showing differences in color, other physical properties, or minor variations in composition from the typical species material. For example, amethyst is the purple variety of the mineral species quartz which has many other varieties as well.
vein:	A zone or belt of mineralized rock lying within boundaries clearly separating it from neighboring rock. It includes all deposits of mineral matter found through a mineralized zone or belt coming from the same source, impressed by the same forms, and appearing to have been created by the same method.
veinlet:	a small vein.
vesicle:	A small cavity in a glassy, igneous rock formed by the expansion of a gas bubble or steam during the solidification of the rock.
vitreous:	Luster similar to freshly broken glass; brightly reflective.

W

Term	Definition
wad:	A dark brown or black, impure mixture of manganese and other oxides. It contains 10 to 20 percent water, and is generally soft; massive and of low specific gravity. Also called black ocher; earthy manganese; bog manganese.
waxy:	Mineral luster that is only slightly reflective; typical of minutely granular surfaces.
wiry:	A mineral occurring as thin wires, often twisted like the strands of a rope, for example native copper.

Source: "California Minerals and Mines" CD-ROM, produced by the California Geological Survey Library. [DMG CD 2000-001, Out of print.]