Calcite

CaCO₃ | Mohs 3 | Hexagonal crystal system

Calcite is a calcium carbonate with perfect rhombohedral cleavage. Optical calcite is famous for it's double refraction properties, splitting incoming light rays into two polarised rays. This property has been used in lenses and prisms. 

Calcite is a polymorph of Aragonite, sharing the same chemistry (CaCO₃).