CRYSTAL SYSTEMS IN GEMOLOGY
There are seven crystal systems based on their crystal symmetry and crystallographic axes (length and direction) and all crystal structures can be assigned to one of these systems.
Each of the crystal systems, except triclinic, has horizontal axes and a vertical crystallographic axis. The direction and length of each axis and the angles between them (alpha, beta, and gamma) are defined by the crystal structure (lattice).
Cubic- four three fold axes- three axes mutually at right angles, and of equal length.
Tetragonal – one vertical four fold axis- three axes mutually at right angles; one axis conventially held vertically, differing in length from the other two.
Orthorhombic- either one two fold axis at the intersection of two mutually perpendicular planes; or three mutually perpendicular two fold axes- three axes mutually at right angles, all of different length.
Monoclonic- one two fold axis- three axes of unequal length; two axes are not at right angles to the plane containing the other two.
Triclinic- either a centre of symmetry or no symmetry- three axes, all of unequal length, none at right angles to the plane containing the other two.
Hexagonal- one vertical six fold axis- four axes, three of equal length, arranged in a horizontal plane; the fourth perpendicular to this plane and of different length from the other three.
