A material is said to be optically isotropic in nature if it shows the same velocity of light in all direction. Glasses and minerals that belong to the isometric system are the rock-forming materials that can be considered isotropic in nature. The chemical bonds in this isotropic materials are the same in all directions, at least on the average, so the electron clouds can oscillate the same in every direction. It has been noted by some authors that the electronic environment that this light sees is independent of the direction of the material.

Materials on the other hand that are showing different velocities in different directions are called optically anisotropic. Those minerals that belong to the tetragonal, orthorhombic, hexagonal, triclinic, and monoclinic system are the rock-forming materials that can be considered anisotropic in nature. Mineral members of these crystal systems are found having lower symmetry than those that belong to the isometric system. Also, these minerals show different strengths of chemical bonding in different directions. The velocity is usually different in different directions because the electron clouds of the atoms or ions are not able to vibrate the same in all directions. However, it should be also noted that if some normally isotropic minerals are unevenly strained, they might be also considered anisotropic.