Iceland spar
（重定向自Iceland crystal）
![Iceland spar, possibly the Icelandic medieval sun stone used to locate the sun in the sky when obstructed from view.[1]](/Images/godic/202501/19/Silfurberg5511.jpg")
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.