mineral

Optical Calcite

Calcium Carbonate (CaCO3)

Hardness: 3 on Mohs scale; Color: Clear to colorless; Luster: Vitreous; Crystal Structure: Trigonal (Rhombohedral); Cleavage: Perfect rhombohedral in three directions; Specific Gravity: 2.71

Hardness: 3 on Mohs scale
Color: Clear to colorless
Luster: Vitreous

Identified July 18, 2026More mineral →

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Physical properties

Hardness: 3 on Mohs scale; Color: Clear to colorless; Luster: Vitreous; Crystal Structure: Trigonal (Rhombohedral); Cleavage: Perfect rhombohedral in three directions; Specific Gravity: 2.71

Formation & geological history

Formed primarily through sedimentary processes by chemical precipitation from calcium-rich solutions, or in hydrothermal veins and metamorphic environments like marble formation.

Uses & applications

Used in the manufacture of high-quality optical instruments during WWII, cement production, acid neutralization, and as a popular specimen for educational mineral collections.

Geological facts

Also known as 'Iceland Spar.' It is famous for its property of double refraction (birefringence), where an object viewed through the crystal appears doubled. Vikings potentially used it as a 'sunstone' for navigation.

Field identification & locations

Identified by its rhombohedral shape, extreme softness (can be scratched by a copper penny), and its reaction (fizzing) when exposed to weak acids like vinegar. Common in limestone regions worldwide.