Here are a few useful terms:
The crystalline system of a gem is the way its atoms are disposed. There are 7 crystalline systems: cubic, hexagonal, quadratic, orthorombic, monoclinic, triclinic and rhomboedric (the former is, for some professionals, a particular case of the hexagonal system).
The refractive index is a value proper to each gem, used to approve the stones. The refraction is the resistance opposed by the material to the crossing of light..
The density, just like the refractive index is very useful to make sure of the genuineness.
La dureté est la résistance dune gemme à la rayure. Elle est définie par léchelle de Mohs de 1 à 10, du talc qui est rayé par tous les minéraux au diamant qui nest rayé que par lui-même.
When referring to isotropic crystal and its opposite anisotropic it refers to an optical characteristic which for the former doesn't vary and for the latter, varies according to the direction of observation. If said biaxial, we are in presence of an anisotropic crystal which has 2 optical isotropic directions. The double refraction is a characteristic of a anisotropic medium which splits the rays that cross it
Asterism is a property of the star-shaped diffusion of light in a gem. Such gems are very often cabochon.