mineral

Optical Calcite (Iceland Spar)

Calcite (Calcium Carbonate, CaCO3)

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

Hardness: 3 on Mohs scale
Color: Colorless to white
Luster: Vitreous to pearly

Identified April 8, 2026More mineral →

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Physical properties

Formation & geological history

Formed through chemical precipitation from calcium-rich solutions in sedimentary environments, hydrothermal veins, or as a major constituent of limestone and marble. Can form in various geological ages ranging from Precambrian to present.

Uses & applications

Used in optical instruments for polarization (historically), as a source of lime for cement, as an educational tool for demonstrating double refraction, and in jewelry/metaphysical collecting.

Geological facts

Famous for its property of 'double refraction' (birefringence), where an image viewed through the crystal appears doubled. It was historically used by Vikings for navigation as a 'sunstone' to locate the sun on cloudy days.

Field identification & locations

Identify by its rhombohedral shape and the double vision effect when placed over text. It will also effervesce (fizz) when in contact with weak acid (vinegar). Found globally, with notable deposits in Iceland, Mexico, and the USA.

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