Glossary of Rock and Mineral Terms - A

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abrasion

A form of mechanical weathering that occurs when loose fragments or particles of rocks and minerals that are being transported, as by water or air, collide with each other or scrape the surfaces of stationary rocks.

abrasive

Hard, tough material used to smooth out rough surfaces; a common abrasive is sandpaper.

absolute hardness

Scale for measuring the hardness of a mineral. The Mohs scale also measures hardness, except the absolute hardness scale has its numbers in proportion. Only scientists use the absolute hardness scale; mineral collectors measure hardness using the Mohs scale.

absorption spectrum

The specific bands of light that pass through and those that get absorbed in a gem or mineral and disperse into the colors of the spectrum. Different gems have different absorption spectra, meaning they are all unique in regard to which colors are absorbed and which pass through. This plays a major role in identifying gems, and can easily distinguish between an imitation gem that has properties almost identical to the real thing. A spectroscope is used to determine the absorption spectra of a mineral.

accessory mineral

Mineral that occurs with other minerals.

accretionary

wedge A mass of sediment and oceanic lithosphere that is transferred from a subducting plate to the less dense, overriding plate with which it converges.

Acicular

An aggregate of long, slender, crystals (i.e. Natrolite). This term is also used to describe the crystal habit of single long, thin, slender crystals.

Acid

Any of a group of chemicals containing a free hydrogen element. Certain acids are used to clean minerals, and tests can be performed on some minerals with certain acids.

acid rain

Rain that contains such acidic compounds as sulfuric acid and nitric acid, which are produced by the combination of atmospheric water with oxides released when hydrocarbons are burned. Acid rain is widely considered responsible for damaging forests, crops, and human-made structures, and for killing aqua-tic life.

acid test

Procedure performed to help identify a mineral. Certain acids (usually hydrochloric acid and nitric acid) are placed in contact with the mineral or its powder, and, depending on the mineral, it dissolves, effervesces, or remains inactive.

activator

Particle in a fluorescent substance that causes the substance to glow when exposed to ultraviolet light.

adamantine

Pertaining to luster. Transparent minerals with a very high luster are said to have an adamantine luster.

adularescence

An effect seen on certain minerals which causes it to display a billowy, rounded, ghost-like reflection with a bluish-whitish color emanating from the surface when the mineral is cut into a cabochon. It is caused by structural anomalies or build up of water in the mineral. The minerals most famous for exhibiting adularescence are Opal and Moonstone, and the name is derived from Adularia, of which Moonstone is a variety of.

aftershock

A ground tremor caused by the repositioning of rocks after an earthquake. Aftershocks may continue to occur for as long as two years after the initial earthquake. The intensity of an earthquake's aftershocks decreases over time.

aggradation

The process by which a stream's gradient steepens due to increased deposition of sediment.

aggregate

A grouping of crystals. Aggregates are defined by the ways crystals are clustered together.

alkaline

Describing a chemical substance that is either a hydroxide, carbonate, or metal oxide that has the ability to turn litmus paper blue, and the ability to react with acids to form salts.

alloy

A metal that is manufactured by combining two or more molten metals. An alloy is always harder than its component metals. Bronze is an alloy of copper and tin.

alloyed

Mixed with another metal to form an alloy.

alluvial fan

A triangular deposit of sediment left by a stream that has lost velocity upon entering a broad, relatively flat valley.

alluvium (alluvial)

Eroded material that gets carried downstream by the current of the stream or river. Alluvial deposits are areas in streams or rivers where alluvium does not continue flowing downstream, thus forming a deposit.

alpine cavity

synonym of vug, a cavity in rock that is lined with long, slender crystals. A vug forms when air pockets form in cooling magma and allow crystals to form in the hollow area

alpine glacier

A mountain glacier that is confined by highlands.

alter

To physically transform from one mineral into another. Process is alteration

amalgam

Any alloy of mercury and another metal. Some amalgams occur naturally. Note: There is a mineral known as Amalgam, and, although it is an amalgam, should not be confused with the term amalgam.

amber

Fossilized pine resin, many times used for ornamental purposes.

amorphous

Without a crystalline shape.

amphibole group (amphibole)

Group of minerals that contain iron, magnesium, silicon, oxygen, and hydroxyl (OH). They may also contain calcium, sodium, and lithium. The amphiboles have prismatic cleavage, and one can note the angle of 56° and 124°, which distinguishes it pyroxenes, which are at 87° and 93° (almost perpendicular). The amphiboles are very similar to the pyroxenes, and the minerals belonging to each group many times cannot be distinguished by ordinary methods. Generally, though, the amphiboles occur in longer, thinner crystals than the pyroxenes. The pyroxenes contain the same elements that the amphiboles do, except they lack hydroxyl, which alters both physical and chemical properties. The amphibole group is in the inosilicate subdivision of the silicates group.

amygdule

A small, bubble-like mass formed from volcanic igneous rock that solidified and a bubble of gas got trapped in the interior.

andesite

The dark, aphanitic, extrusive rock that has a silica content of about 60% and is the second most abundant volcanic rock. Andesites are found in large quantities in the Andes Mountains.

angle of repose

The maximum angle at which a pile of unconsolidated material can remain stable.

anhydrous

Without water. Anhydrous minerals contain no water in their chemical structure. The term anhydrous is usually used in reference to an anhydrous mineral belonging to a group which contains many hydrous minerals.

anion

Neutral atom that gains an electron and becomes negatively charged.

anisotropic

Lacking consistent hardness on all surfaces. For example, the hardness of Kyanite on the Mohs scale is between 6 and 7 lengthwise, and between 4 and 4½ crosswise

anthracite

A hard, jet-black coal that develops from lignite and bituminous coal through metamorphism, has a carbon content of 92% to 98%, and contains little or no gas. Anthracite burns with an extremely hot, blue flame and very little smoke, but it is difficult to ignite and both difficult and dangerous to mine.

anticline

A convex fold in rock, the central part of which contains the oldest section of rock. See also syncline.

antimonides

Group of sulfides that contain one or more true metals combined with the semi-metal antimony.

aqua fortis

Synonym of nitric acid

aqua regia

HCl),(HNO3) Mixture of hydrochloric and nitric acids. It is an extremely destructive mixture and can dissolve gold and platinum, as well as many other minerals.

aqueous

Formed from precipitating hard water. Stalagmites and stalactites are common examples.

aquiclude

An impermeable body of rock that may absorb water slowly but does not transmit it.

aquifer

A permeable body of rock or regolith that both stores and transports groundwater.

aquitard

A layer of rock having low permeability that stores groundwater but delays its flow.

arragonite group

Group of minerals belonging to the carbonate group that are isomorphous with one another. They all crystallize in the orthorhombic system, exhibit good cleavage (although not as good as the members of the calcite group), have a weak double refraction in transparent specimens, and commonly intergrow in three individuals forming six sided trillings.

arborescent

Synonym to dendritic: Aggregate composed of skeletal or tree-like formations. May be a single entity, or a formation that forms from mineral-rich solutions that deposit the mineral in rock and form a tree or plant structure embedded in rock. There sometimes is a distinction noted between the two aggregates; in some guides the former aggregate (single entity) is known as skeletal, and the latter as dendritic (embedded in rock). Other guides, such as this one, don't distinguish the two, and term them both as dendritic

arête

A sharp ridge of erosion-resistant rock formed between adjacent cirque glaciers.

argentiferous

Containing silver.

argillaceous

Composed mostly of clay.

arid

A dry, desert region. Many minerals that exist only in arid regions are usually the result of evaporation.

aridity

index The ratio of a region's potential annual evaporation, as determined by its receipt of solar radiation, to its average annual precipitation.

arroyo

A small, deep, usually dry channel eroded by a short-lived or intermittent desert stream.

arsenates

Group of minerals that are compounds of one or more metallic elements associated with the arsenate radical (AsO4). The arsenates, together with the related vanadates, are classified in the phosphate group. Most arsenates are heavy, and none are hard. They are usually brittle and occur in small crystals or compact aggregates.

arsenides

(subgroup) Group of sulfides that contain one or more true metals combined with the semi-metal arsenic.

artesian

Of, being, or concerning an aquifer in which water rises to the surface due to pressure from overlying water.

asbestos

Fibrous minerals of the amphibole group, as well as fibrous Serpentine, are known as asbestos.

asbestos amphibole

Any extremely fibrous mineral of the amphibole group

asterism

Effect exhibited on some minerals (usually only in polished cabochons) causing it to reflect a billowy, star-like formation of concentrated light which moves around when the mineral is rotated. Asterism is caused by dense inclusions of tiny, parallel, slender, fibers in the mineral which cause the light to reflect in such an interesting manner. Minerals that display asterism may exhibit four, six, and sometimes twelve rayed "stars", depending on the inclusions, size, and facet mode. Some specimens may display much stronger asterism than others, and some specimens may have areas where the inclusions are not present, leaving holes or empty areas in the star.

asteroid

Large solid mass suspended in outer space that revolves around the sun. Asteroids are larger than meteoroids.

asthenosphere

A layer of soft but solid, mobile rock comprising the lower part of the upper mantle from about 100 to 350 kilometers beneath the Earth's surface. See also lithosphere.

asymetrical

Not containing perfect symmetry.

atoll

A circular reef that encloses a relatively shallow lagoon and extends from a very great depth to the sea surface. An atoll forms when an oceanic island ringed by a barrier reef sinks below sea level.

atom

The smallest particle that retains all the chemical properties of a given element.

atomic mass

1. The sum of protons and neutrons in an atom's nucleus. 2. The combined mass of all the particles in a given atom.

atomic number

The number of protons in the nucleus of a given atom. Elements are distinguished from each other by their atomic numbers.

atomic structure

The arrangement and the type of atoms that exist in a particular substance.

auriferous

Containing gold.

Autochthonous

Describing a rock or mineral that formed in the same location where it was found.

adventurescence

The effect caused by small inclusions of a mineral with a highly reflective surface (commonly Hematite, Pyrite, or Goethite) which causes it to exhibit a glistening effect, as if it is pasted with glitter, when rotated or looked at different points. The name is derived from Aventurine, a green variety of Quartz that exhibits this effect.

aventurization

The effect caused by small inclusions of a mineral with a highly reflective surface (commonly Hematite, Pyrite, or Goethite) which causes it to exhibit a glistening effect, as if it is pasted with glitter, when rotated or looked at different points. The name is derived from Aventurine, a green variety of Quartz that exhibits this effect.

axis

Imaginary line drawn through the center of an object, either horizontally or vertically. In the case of minerals, it is used to determine if and how mineral has symmetry. The horizontal axis is known as the x axis, the vertical axis as the y axis. Axis lines are usually drawn as dotted lines. Plural is axes.

axis of symmetry

An imaginary line drawn through the center of a crystal that replicates the exact shape if the crystal is turned to 360°. Minerals can have symmetry on their x axis (horizontal symmetry), on their y axis (vertical symmetry), and both their x and y axes. Symmetry can also be calculated on degrees, which depends on how often a crystal must be rotated to bring out symmetry (i.e. a hexagon has six-fold symmetry on a 60° angle)