Calcite, Calcium Carbonate (CaO3)
Calcite is a natural crystal of calcium carbonate, with a hexagonal-rhombohedral structure. With a high birefringence, wide spectral transmission and wide availability of sizes it has been a popular choice for visible and near-IR polarisation optics.
However, calcite is also susceptible to moisture and has a low hardness (Mohs hardness is 3) which makes obtaining good optical quality crystals relatively difficult. For critical applications materials such as yttrium vanadate are replacing calcite but for low cost applications calcite remains a good choice.