Calcite (_CaCO3) is one of the most stable polymorphic calcium carbonates. On the Mohs hardness scale, this material defines hardness 3, i.e. it can be scratched with a copper coin. It dissolves in slightly acidic solutions. Water and_CO2 from the environment are sufficient for easy decomposition. Transparent calcite is interesting because of its strong birefringent properties.

In optics, pure calcite in particular is used for polarizers (e.g. as a Glan-Foucault prism) or as a wave plate (λ/n plate) due to its strong birefringence.