Doubly refracting Calcite from Iceberg claim, Dixon, New Mexico. This 35 pound (16 kg) crystal, on display at the National Museum of Natural History, is one of the largest single crystals in the United States.

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).^[1][2] It occurs in large readily cleavable crystals, easily divisible into rhombs, and is remarkable for its double refraction.^[3] Historically, the phenomena of this crystal have been studied at length by Huygens and Newton.^[4] 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.^[5][6]

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.^[7]

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.

References

Source: adapted by the editor from Wikipedia, the free encyclopedia; from the article "Iceland spar". Image Credit.

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