Iceland Spar (Optical Calcite)

Physical Properties

Hardness: 3 on Mohs scale; Color: Colorless/transparent; Luster: Vitreous; 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 most famous for forming in volcanic cavities as large, clear crystals.

Uses & Applications

Historically used in optical instruments (microscopes, gun sights) for its polarizing properties. Today it is used for geological teaching, spiritual/reiki collecting, and as a source of high-purity calcium carbonate.

Geological Facts

Iceland Spar is famous for the property of double refraction (birefringence). If you place it over text, the text will appear double. Some historians believe the 'Sunstone' used by Vikings for navigation was Iceland Spar.

Field Identification & Locations

Identify in the field by its perfect rhombohedral shape and double refraction. It will also effervesce (fizz) vigorously when in contact with weak acid like vinegar. Commonly found in Iceland, Mexico, and China.

Identified on: 4/13/2026

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