mineral

Optical Calcite (Iceland Spar)

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

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

Formation & geological history

Formed through sedimentary precipitation in marine environments, or in hydrothermal veins and caverns. It is found in geological formations ranging from the Archean eon to the present.

Uses & applications

Used in the manufacture of cement and mortar, as a soil conditioner in agriculture, in the optical industry for polarizing filters (due to double refraction), and as a metaphysical/collector specimen.

Geological facts

Optical calcite exhibits 'double refraction,' meaning that if you place it over a piece of text, you will see two images of each letter. It was historically used by Vikings as a 'Sunstone' for navigation on cloudy days.

Field identification & locations

Identify in the field by its rhombohedral cleavage (it breaks into slanted cubes) and its rapid effervescence (fizzing) when a drop of dilute hydrochloric acid is applied. Very common in limestone and marble regions worldwide.

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