mineral

Calcite (Optical Grade/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 March 30, 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 hydrothermal veins. Found in various geological ages from Precambrian to Holocene. This specific rhombohedral form often results from the cleavage of larger crystals.

Uses & applications

Used in the manufacture of cement and mortar, as an agricultural soil treatment, and historically in optical instruments (like polarizing microscopes) due to its high birefringence.

Geological facts

Calcite is famous for its 'double refraction' property; if you place a clear crystal over text, the text will appear doubled. It is the primary component of limestone and marble.

Field identification & locations

Identify in the field by its rhombohedral shape and its reaction to cold dilute hydrochloric acid (it will fizz/effervesce). It is common globally, with significant deposits in Iceland, Mexico, and the USA (Missouri). Collectors should look for clean cleavage planes.

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