Iceland Spar

Iceland spar, formerly known as Iceland crystal (Icelandic: silfurberg; 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, easily divisible into rhombs, and is remarkable for its double refraction. Historically, the phenomena of this crystal were studied at length by Christiaan Huygens and Isaac Newton. Sir George Stokes also studied the phenomenon.

Mines producing Iceland spar include many mines producing related calcite and aragonite as well as 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 the People's Republic of China.