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.

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Iceland spar [′īs·lənd ′spär]

(mineralogy)

A pure, transparent form of calcite found particularly in Iceland; easily cleaved to form rhombohedral crystals that are doubly refracting. Also known as Iceland crystal.

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Iceland Spar 

(limestone spar), a variety of the mineral calcite consisting of transparent, colorless, or slightly tinted crystals. It is used in constructing polarizing prisms for various instruments producing polarized light. There is a large deposit near Helgustadir at Eskifjordur in Iceland (hence the name), where optical-quality crystals have been extracted from cavities in basalt. In the USSR deposits of Iceland spar have been found in the Nizhniaia Tunguska Basin, the Yakut and Tuva ASSR’s, the Northern Caucasus, Transcaucasia, Middle Asia, and the Far East.