CRYSTAL SYMMETRY IN GEMOLOGY

Crystals can be classified into groups depending upon their symmetry, which in turn is defined by the degree of regularity in the arrangement of the atoms in the crystal structure. This arrangement also affects their optical and physical properties, for example how they react to light, the way they look and feel, and their hardness.

The main method by which a gemologist usually defines the symmetry of a crystal is by reference to its planes of symmetry and axes of symmetry (sometimes referred to as mirror symmetry and rotational symmetry).

plane of symmetry

A plane of symmetry is an imaginary plane (or mirror) that divides a crystal such that the image on one side of the plane is the mirror image of that on the other side.

AXIS OF SYMMETRY

An axis of symmetry is an imaginary line (or thread) that runs through a crystal and about which the crystal can be rotated, in such a way that it looks the same two, three, four, or six times during a complete circle (rotation) of 360 degrees.

crystallographic axis (reference axis)

A crystal is also defined by its reference or crystallographic axes. The crystallographic axis is an imaginary line that runs through a crystal and indicates both the direction and length of the repeating pattern of the atoms (the lattice structure). The lengths of the crystallographic axes are proportional to the repeat directions in the three principal directions (a, b, and c) in the crystal structure or lattice (x, y, and z). Crystallographic axes are not the same as the axes of symmetry, although they may run parallel to each other.