This section is covered in Chapter 4 of Nesse.
In Isotropic Materials - the velocity of light is the same in all directions. The chemical bonds holding the material together are the same in all directions, so that light passing through the material sees the same electronic environment in all directions regardless of the direction the light takes through the material.
Isotropic materials of interest include the following isometric minerals:
If an isometric mineral is deformed or strained then the chemical bonds holding the mineral together will be effected, some will be stretched, others will be compressed. The result is that the mineral may appear to be anisotropic.