Not every direction in a doubly refractive stone is a direction of double refraction. Instead, every such stone, depending on the crystal system to which it belongs, has one or two directions of single refraction; i.e., directions in which light is not polarized. These directions are referred to as OPTIC AXES. If light enters a doubly-refractive material at such an angle that it is refracted along a path parallel to its optic axis, it is not broken into two beams but is refracted as a single beam, just as though it had been refracted into an amorphous material or a mineral of the cubic system. Gems of the tetragonal and hexagonal systems have one such direction of single refraction, or optic axis (UNIAXIAL), whereas those stones of the orthorhombic, monoclinic and triclinic systems have two optic axes (BIAXIAL).
The reason for the existence of an optic axis is explained by Figure 6. This sketch shows a simple crystal form of the tetragonal system and a sphere cut from it, with vertical and horizontal lines drawn in to illustrate, basically, the vibration planes in the material; i.e., the planes in which the vibration of light passing through it are confined. The horizontal planes are parallel to one another, whereas the vertical planes converge at a central point. Obviously, light passing through the crystal in any direction other than parallel to the optic axis will encounter just two planes of vibration, which are at right angles to one another. On the other hand, light transmitted parallel to the optic axis will encounter an infinite number of these vibration planes radiating out from a common center. Thus a ray of light vibrating in all directions will not be affected as it travels through the stone, and will therefore emerge still vibrating in all directions, as through it had passed through a singly-refractive material.
The vibration planes in biaxial crystals (those in the orthorhombic, monoclinic and triclinic systems) are somewhat more complicated than those of uniaxial crystals, since there are two directions in which the vibration planes converge.