2.2.1 Isotropic and anisotropic materials
Materials that have the same atomic structure in all directions are termed isotropic. When light passes through such a material it doesn't matter in which direction the light vibrates, its speed (and therefore the refractive index) is the same. This is true of light passing through air, water, glass and some minerals.
To which crystal system would you expect an isotropic mineral to belong?
The cubic system, because it defines materials for which the crystal structure and, consequently, its optical properties are the same along each crystallographic axis.
Minerals belonging to other crystal systems are more common than those with cubic symmetry. They have internal structures that are not the same along all crystallographic axes and are therefore anisotropic minerals. The importance of this is that the behaviour of the light passing through an anisotropic crystal depends on the vibration direction of the light and its relationship to the crystal structure.