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, formerly known as Iceland crystal, is a transparent variety of calcite, or crystallized calcium carbonate, originally brought from Iceland, and used in demonstrating the polarization of light (see polarimetry). It occurs in large readily cleavable crystals, easily divisible into rhombs, and is remarkable for its double refraction. Historically, the phenomena of this crystal have been studied at length by Huygens and Newton. Sir George Stokes also studied the phenomenon.

Viking "sunstone"

It has been speculated that the Vikings used the light-polarizing property of Iceland spar, which they called sunstone (a different mineral than the gem-quality sunstone), to tell the direction of the sun on cloudy days, for navigational purposes.

In 2007, Gabor Horvath from Eotvos Lorand University in Budapest, Hungary confirmed that the polarization of sunlight in the Arctic can be detected using a sunstone.

In literature

Thomas Pynchon references the doubling property of Iceland spar in his 2006 novel Against the Day. A section of the novel is entitled "Iceland Spar".

Phillip Pullman references the doubling property of Iceland spar in his 2000 novel The Amber Spyglass, the third volume in the His Dark Materials trilogy.

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