Light traveling along the direction of this single optic axis exhibits the same properties as isotropic materials in the sense that the polarization direction of the light is not changed by passage through the crystal. The single optic axis is coincident with the c-crystallographic axis in tetragonal and hexagonal minerals. Like all anisotropic substances, the refractive indices of uniaxial crystals varies between two extreme values. Values between w and e are referred to as e ' . Uniaxial minerals can be further divided into two classes.
All anisotropic minerals exhibit the phenomenon of double refraction.
Only when the birefringence is very high, however, is it apparent to the human eye. Calcite has rhombohedral cleavage which means it breaks into blocks with parallelogram - shaped faces. If a clear rhombic cleavage block is placed over a point and observed from the top, two images of the point are seen through the calcite crystal. This is known as double refraction. What happens is that when unpolarized light enters the crystal from below, it is broken into two polarized rays that vibrate perpendicular to each other within the crystal.