Iceland spar

Iceland spar, colorless variety of crystallized calcite , characterized by its properties of transparency and double refraction. It is used chiefly in the manufacture of Nicol prisms , which are essential parts of polarizing microscopes and other optical instruments. The principal deposit is in Iceland, but small quantities are found in other countries, including the United States and Mexico. Iceland spar is believed by some to have been used by the Vikings as a sunstone .

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