Iceland spar
Iceland spar, formerly known as Iceland crystal ( ; lit. "silver-rock"), 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, is easily divisible into rhombuses, and is remarkable for its birefringence. This means that the index of refraction of the crystal is different for light of different polarization. A ray of unpolarized light passing through the crystal divides into two rays of perpendicular polarization directed at different angles, called double refraction. So objects seen through the crystal  appear doubled. Historically, the double-refraction property of this crystal was important to understanding the nature of light as a wave. This was studied at length by Christiaan Huygens and Isaac Newton. Sir George Stokes also studied the phenomenon. Its complete explanation in terms of light polarization was published by Augustin-Jean Fresnel in the 1820s. Mines producing Iceland spar include many mines producing related calcite and aragonite as well as those famously in Iceland, productively in the greater Sonoran desert region as in Santa Eulalia, Chihuahua, Mexico and New Mexico, United States, as well as in China. The clearest  specimens, as well as the largest, have been from the Helgustadir mine in Iceland. Viking "sunstone". It has been speculated that the sunstone (, a different mineral from the gem-quality sunstone) mentioned in medieval Icelandic texts was Iceland spar, and that Vikings used its light-polarizing property to tell the direction of the sun on cloudy days for navigational purposes. The polarization of sunlight in the Arctic can be detected, and the direction of the sun identified to within a few degrees in both cloudy and twilight conditions using the sunstone and the naked eye. The process involves moving the  stone across the visual field to reveal a yellow entoptic pattern on the fovea of the eye, probably Haidinger's brush. The recovery of an Iceland spar sunstone from an Elizabethan ship that sank in 1592 off Alderney suggests that this navigational technology may have persisted after the invention of the magnetic compass. Nicol prism. William Nicol (1770–1851) invented the first polarizing prism, using Iceland spar to create his Nicol prism. Cultural references. The Thomas Pynchon novel "Against the Day" uses the doubling effect of Iceland spar as a theme.