1. refractive index not depend on the direction isotropic materials
2. refractive index does depend on the direction that the light travels anisotropic materials - have a range of refractive indices
Minerals crystallize in the tetragonal and hexagonal crystal systems -uniaxial *2 extreme refractive indices for each wavelength.
Minerals crystallize in the triclinic, monoclinic, and orthorhombic crystal systems -biaxial *characterized by 3 refractive indices