A number n found by dividing the velocity of light in a vacuum c by the velocity of light in a transparent substance. Isometric crystals are isotropic and have one index of refraction; all other symmetries are anisotropic. Hexagonal, trigonal, and tetragonal crystals have two principal indices; orthorhombic, monoclinic, and triclinic crystals have three principal indices. Determination of indices of refraction of a mineral is a major means of mineral identification.