Anisotropic

The speed of light through any medium is inversely proportional to the refractive index of the medium. The refractive index of a mineral is determined by the arrangement and type of atoms within its structure.

If the structure of the mineral allows light to travel at different velocities depending on the direction in which the light is passing through it, the mineral is anisotropic. That is to say, it has different refractive indices in different orientations.