What does isotropic mean in mineralogy ?

Isotropic : definition

Said of the quality of a material whose physical characteristics remain identical in all directions of space. In crystalline optics, this property exists in cubic (garnet for example) and amorphous (opal, glass, etc...) minerals. It results in a total extinction (black color in all directions) under a polarizing microscope in analyzed polarized light (see photos on right of garnets - P. Jauzein) and therefore by the absence of birefringence, pleochroism, etc. This term is the opposite of anisotropic.