Absolute hardness
It is a scale to measure the hardness of a mineral. It is used by scientists as it has its numbers in proportion. Mineral collectors measure hardness using the Mohs scale.

Absorption spectrum
It is the absorbed light in a gem or mineral when bands of light passes through. Different gems have different absorption spectra. All are unique about which colors are absorbed and which pass through. This plays a major role in identifying gems and it is easily distinguished between an imitation gem to the real thing. To determine the absorption spectra of a mineral, a spectroscope is used.

Accessory mineral
It is the mineral, which occurs with other minerals.

Acid test
It is a process of identifying a mineral. Certain acids, like hydrochloric acid and nitric acid, are brought in contact with the mineral or its powder and it dissolves, effervesces, or remains inactive, depending on the mineral.

Adamantine (luster)
It is a transparent mineral having a very high luster.

Adit
It is a term used for common mining of a horizontal to sub-horizontal tunnel driven into a hillside to access an ore body.

Agglomerate
It is a volcanic rock that consists of fragments of pyroclastic rocks, more than 2 cm in size.

Alkaline
It is a term which describes a chemical substance, either a hydroxide, carbonate, or metal oxide. They have the ability to turn litmus paper blue, and to form salts after reacting with acids.

Alter
It is a term for transforming one mineral into another, physically.

Alteration
It is the process where a rock or a mineral is transformed into another one.

Amphibole group
It is a group of minerals containing iron, magnesium, silicon, oxygen, and hydroxyl (OH). They also sometimes contain calcium, sodium, and lithium. Amphiboles have prismatic cleavage. The angles are 56° and 124°, which distinguishes it pyroxenes that is 87° and 93°, almost perpendicular. Both are so similar that many times it cannot be distinguished by ordinary methods. Though amphiboles usually occur in longer, thinner crystals than the pyroxenes, but pyroxenes contains similar elements, except they lack hydroxyl that alters both the physical and chemical properties. The amphibole group is in the inosilicate subdivision of the silicates group.

Amygdule
It is a small, bubble-like mass which is formed from the volcanic igneous rock that solidifies and bubble of gas get trapped in the interior.

Anhydrous
It simply means without water. The anhydrous minerals do not contain water in their chemical structure. The term is generally used in reference to an anhydrous mineral which belongs to a group containing many hydrous minerals.

Anisotropic
One which lacks consistent hardness on all surfaces, like the hardness of Kyanite on the Mohs scale is 6 to 7 lengthwise, and 4 to 4 1/2 crosswise.

Anticline
It is a fold in a rock where the strata protrudes upwards towards the fold. Each section of the rock reaches the highest point by the fold area.

Aragonite group
It is a group of minerals which belongs to the carbonate group that are isomorphous with one another. All of them crystallize in the orthorhombic system, exhibits good cleavage, not like calcite group, have a weak double refraction in a transparent specimens, and they commonly intergrows in three individuals forming six sided trillings.

Argillacious
One that is mostly composed of clay.

Arid
It is a term used for a dry, desert region. Many minerals which exist in arid regions are the result of evaporation.

Arsenates
It is a group of minerals which are compounds of one or more metallic elements associated with the arsenate radical, AsO₄. The arsenates with the related vanadates are classified in the phosphate group. Most of them are heavy but not a single one is hard. They are generally brittle and occurs in small crystals or compact aggregates.

Arsenide
It is a mineral which is formed by the combination of arsenic with another chemical like sperrylite, PtAs₂.

Asbestos
It is the fibrous minerals of the amphibole group. Fibrous serpentine is also known as asbestos.

Asbestos amphibole
It is any extremely fibrous mineral from the amphibole group.

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

Aventurescence
It is the effect being caused by small inclusions of a mineral having a highly reflective surface that causes it to exhibit a glostening effect when rotated or looked at different points. The common examples are Hematite, Pyrite, or Goethite. The name has been derived from Aventurine, which is the green variety of Quartz that exhibits this effect. The effect is also called as aventurization.

Axis
It is an imaginary line, either horizontal or vertical, drawn through the center of an object. For minerals, it is used in determining the 'if' and 'how' the mineral has symmetry.