Iceland Spar is a transparent, exceptionally pure variety of calcite, calcium carbonate (CaCO3), best known for its dramatic double refraction. Light entering the crystal splits into two rays, so any object viewed through it appears doubled—a striking optical effect caused by calcite’s strongly anisotropic crystal structure. This property is so pronounced that Iceland spar became historically important in early studies of polarization and optics, helping scientists understand how light behaves inside crystals. It forms in the trigonal crystal system and has a Mohs hardness of just 3, which means it scratches easily despite its glassy clarity. Large clear pieces were famously collected in Iceland, which gave the mineral its name, though it also occurs elsewhere in hydrothermal veins and limestone environments. Scientifically, Iceland spar is a standout example of how crystal chemistry can control light itself: one mineral, one composition, but an optical effect so strong it helped shape modern physics. Each specimen is approximately a 1" cube. Size, appearance, and optical effect may vary based on natural formation.
