mineral

Optical Calcite (Iceland Spar)

Calcite (Calcium Carbonate, CaCO3)

Hardness: 3 (Mohs scale); Color: Colorless to white/translucent; Luster: Vitreous to pearly; Crystal structure: Trigonal/Rhombohedral; Cleavage: Perfect rhombohedral in three directions; Specific gravity: 2.71

Hardness: 3 (Mohs scale)
Color: Colorless to white/translucent
Luster: Vitreous to pearly

Identified March 12, 2026More mineral →

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Physical properties

Formation & geological history

Formed through sedimentary precipitation in marine environments or as an evaporite; also found in hydrothermal veins and metamorphic marble. Its geological age varies from Precambrian to Holocene.

Uses & applications

Used in optical instruments for polarizing light, as a decorative collector's specimen, in the manufacture of cement, as a soil conditioner in agriculture, and as a source of calcium in the chemical industry.

Geological facts

Optical calcite is famous for its double refraction (birefringence). If you place a clear crystal over a line of text, you will see two images of the same line. Historically, it may have been the 'Sunstone' used by Vikings for navigation on cloudy days.

Field identification & locations

Identify in the field by its rhombohedral shape (like a leaning box) and its strong reaction (fizzing) to dilute hydrochloric acid. It is commonly found globally, with famous deposits in Iceland, Mexico, and the USA. Collectors look for high clarity and well-defined rhombohedral cleavage.

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