Calcite
The most common form of calcium carbonate, calcite is known for the great variety and beautiful development of its crystals. Highly transparent rhombohedra calcite are known as optical spar, a reference to their use in polarizing filters. Calcite perfectly demonstrates the optical property of double refraction. Although it forms spectacular crystals, most calcite is massive, occurring either as limestone or marble. Well known deposits of calcite can be found in Iceland, Mexico, USA, China, and Madagascar.
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