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 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.