A term sometimes applied to rock crystal and a term sometimes applied to crystal glass.A body formed by the solidification of certain elements or compounds and having a definite external form, bounded by natural plane surfaces symmetrically arranged, which is the external expression ofa definite external form, bounded by natural plane surfaces symmetrically arranged, which is the external expression of a definite internal structure owing to atomic arrangement.Gems of greatest value are those cut from large transparent crystals, but some are from minerals composed of small crystals or without crystalline structure.Crystals are classified into seven systems according to their symmetry and also have characteristic forms (called 'crystal habit'), but the technical details of such systems and habits are beyond thescope of this site, which deals primarily with antique jewelry.