Calcite (CaCO3) belongs to the most stable polymorph calcium carbonates. It defines the Mohs hardness scale with 3 as the hardness. That means that the material can be scratched with a copper coin. It dissolves in acids where ambient CO2 and water is enough to show slight dissolution. Transparent calcite is interesting because of its strong birefringence.
As a result, calcite is used for polarizers (e.g., as Glan-Foucault prism) and for waveplates (λ/n plates).