Welcome to the Mineral and precious stone glossary

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z  All terms

Crystal habit describing a flat, tough, usually four sided crystal.

Property in certain minerals that cause them to discolor when placed in certain environments. A famous example is Silver, which develops a yellowish-black coating on its surface when exposed to hydrogen sulfide (H2S), which is present in small quantities in the atmosphere. See also oxidize.

Group of silicate minerals that have all four oxygen atoms from each tetrahedron share itself with an adjacent tetrahedron.

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

The reaction of a mineral to certain stress. (See Tenacity in mineral properties for more information.)

The end of a crystal face, usually referring to its base.

Any mineral that falls under the following specifications belongs to the tetragonal crystal system: Three axes, two are equal in length, one is unequal. All three axes are at 90° to each other.

Exhibiting the shape of a tetrahedron.

The building block of all silicates. It is composed of one silicon atom equidimensionally placed around four oxygen atoms. The groups of silicates are classified based on how the tetrahedrons join together.

The feel and appearance of a mineral, such as color, crystal form, and luster.

Phenomenon exhibited in certain minerals that they give off light, like a glow, when heated. Can only be observed in a dark area.

Emitting light after being heated.

The alteration of one mineral to another.

The quality of how a mineral lets light through.

Describing an object that is able to transmit light through it, and if placed in front of another object, that object can be seen as if there is nothing interfering.

Describing something that is able to transmit light but not fully. Objects can be seen through a translucent object, but they will be unclear.

Rock formed from solidified lava, which usually contains pockets holding crystals.

Property exhibit in certain minerals which causes them to give off orange or yellow "flashes" when sawed or struck.

Any mineral that falls under the following specifications belongs to the triclinic crystal system: Three axes, all of them are unequal in length, none of them are right angles to each other.

The trigonal crystal system is really part of the hexagonal crystal system and only has one minute difference (in that true hexagonal minerals have sixfold symmetry, whereas trigonal minerals have threefold symmetry). The trigonal crystal system, like the hexagonal crystal system, has: four axes, three are equal in length and lie at an angle of 120° from each other. The fourth is either longer or shorter but must be at a right angle toward the other corners.

Intergrowth of three orthorhombic crystals that twin at the center and form a hexagonal shaped crystal

Gem composed of three cross-sections pasted together. The top portion, which is the portion that is visible, consists of a naturally occurring, valuable gem, while the bottom two portions are low-grade material, such as glass or rock crystal. See also doublet.

24 sided polyhedron; all sides are equidimensional. Minerals shaped as trisoctahedrons belong to the isometric system. Plural = Trisoctahedra or Trisoctahedrons

An individual portion of a vein.

Complex, scientific test which is conducted to identify a mineral. A mineral is crushed and its powder is placed in an open or closed tube, which is then heated; the reaction of the powder in the tube to the heat acts as a guide for identifying the presence of certain metals in the mineral.

Aggregate composed of large, oval, "bubbles" protruding from a surface. Similar to pisolitic, but "bubbles" are protruding instead of slightly rounded. (i.e. Aragonite)

Spring which produces tufa.

Porous igneous rock composed of volcanic ash compacted together.

Having been placed in a tumbler, which smoothes out and enhances the luster of a mineral. Rocks and minerals may be naturally tumbled, if they were smoothed and rounded from having been in a fast-flowing stream or river for a long period of time.

Machine that smoothes the surfaces and increases the luster of minerals by "tumbling" them together in a cylindrical container that rotates, causing them to continually bang into one another.

Group of minerals that are compounds of one or more metallic elements and the tungstate radical (WO4). These minerals are very heavy, soft, and brittle. The tungstates are usually categorized with the chemically related molybdates, in which they can be partially replaced by. The tungstates are also known as the wolframates.

Two or more crystals that intergrow in a specific method. May also be used in verb form, as "Two crystals may twin to form a cross." See Crystals in mineral properties for more information.

Tendency of some crystals to intergrow in a distinct way or form specific, repeated patterns. See penetration twinning, repeated twinning, and contact twinning.