Glossary

Allochromatic : qualifying gems coloured by impurities without which they would be of another color.

Amorphous : without regular atomic structure nor geometrical external form.

Birefringence (B) : for the gems with double refraction, difference between the higher and lower refractive index

Brilliance : brightness of a stone following the reflexion of the light on its surface.

Cabochon : stone cut and polished in dome.

Carat : unit of weight used for the gems. 1 carat = 0.20 gram

Cleavage : crack of a stone along lines of less resistance, determined by the atomic structure.

Crystal : solid whose atoms are distributed in space with a geometrical regularity.

Crystal system : classification of different atomic structure of a crystal.

Dichroic : property of a gem to appear of two colors or nuances according to the angle of sight.

Double refraction : phenomenon in virtue of which any luminous ray is divided into two when it penetrates in an anisotropic ore.

Fracture : crack of a stone without direct relationship to the atomic structure.

Gem : this term groups together the precious and semi-precious stones. Some substances of organic origin (amber, coral, pearl,…) are comparable with the gems.

Idiochromatic : qualifying gems whose color is due to components of their chemical composition and not to impurities.

Inclusion : foreign body inside a gem.

Lapidary : craftsman who works or sells semi-precious or precious stones except for diamonds (diamond cutter).

Mineral : natural inorganic matter characterized by chemical properties and determined physics, which can be crystallized or amorphous.

Mohs scale : cale of measurement of the relative hardness of the minerals (classification from 1 to 10).

Ore: rock containing of metals which one can extract with profit.

Precious stone : diamond, emerald, ruby and sapphire.

Refractive index (RI) : measure deceleration and change of direction of the penetrating luminous rays in a gem.

Rock : matter made up of one or more minerals.

Semi-precious stone : stone which does not belong to the category of the precious stones.

Specific gravity : measure density of a gem.

Trichroic : property of a gem to appear of three colors or nuances according to the angle of sight.

Twins : grouping of several homogeneous crystals, juxtaposed or interpenetrated according to geometrical laws well defined.