Iceland Spar (Optical Calcite)

Physical Properties

Hardness: 3 on Mohs scale; Color: Colorless/Transparent; Luster: Vitreous to Pearly; Crystal Structure: Trigonal/Rhombohedral; Cleavage: Perfect rhombohedral in three directions; Specific Gravity: 2.71

Formation & Geological History

Formed through chemical precipitation in sedimentary environments or hydrothermal veins. This variety is noted for its high purity and transparency, often found in volcanic cavities or limestone deposits.

Uses & Applications

Used in optical instruments for polarization, historical use in navigation (sunstone theory), as a source of calcium in industry, and highly valued by mineral collectors for its double refraction.

Geological Facts

Iceland Spar is famous for its property of double refraction (birefringence); when light passes through it, it splits into two rays, making objects behind it appear doubled. It was historically used by Vikings for navigation on cloudy days.

Field Identification & Locations

Identify by its perfect rhombohedral cleavage (tilted cube shape) and its ability to double images when placed over text. Found globally with major deposits in Iceland, Mexico (Chihuahua), and the USA.

Identified on: 5/11/2026

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