Calcite is famous for producing double images through strong birefringence.
A clear variety of calcite called Iceland spar splits light into two rays traveling at different speeds. Looking through a clear piece can therefore make a single printed line or object appear doubled. This optical behavior results from calcite’s anisotropic crystal structure, which gives light different refractive properties in different directions.
Calcite is calcium carbonate, CaCO₃, and is the main mineral in limestone and marble. It is also a major component of shells and many marine sediments. Iceland spar was historically important in the study of optics, while modern calcite is used in some polarizing instruments. Double refraction is not the same as ordinary reflection or simply seeing a transparent object through a crack.