15. Optic Axis
This is more commonly used to identify the rotational symmetry of any optical systems such as a camera lens or microscope. In gemmology, however, the optic axis of a crystal indicates the direction in which a ray of transmitted light passes through it without forming a double refraction. A crystal’s optic axis is always a direction, never a line, and crystals may possess either a single optic axis (uniaxial) or two different optic axes (biaxial). Only crystals have an optic axis as non-crystalline materials in nature generally don’t create the double refraction of light.
