Crystals are divided in to 7 systems, each corresponding to a specific set of angular, geometrical, and symmetrical specifications. Every gem group can be classified according to its crystallization. The study of crystallization is invaluable in the determination and identification of rough gem materials.

monoclinic cubic tetragonal orthorhombic triclinic trigonal hexagonal

Crystals are formed by a unit cell which repeats in 3 dimensions. The symmetry of this cell occurs in 7 distinct systems, depending the cell shape and symmetry. Symmetry is influenced by the cell shape (eg. a non-cubic cell will never have cubic symmetry) and the layout of the atoms within the cell. Small (but regular) displacements of atoms within a cell due to temperature and/or pressure, can and do change the symmetry of a mineral. The cell dimensions are usually named "a,b,c". Where one is of different length to the other or is otherwise "special", this is usually labelled "c".