mineral

Optical Calcite (Iceland Spar)

Calcium Carbonate (CaCO3)

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

Identified March 16, 2026More mineral →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed through chemical precipitation in sedimentary environments, hydrothermal veins, or as a primary mineral in igneous carbonatites. Can be found in geological formations ranging from the Archean eon to modern day.

Uses & applications

Used in high-precision optical instruments for its double refraction properties, as an abrasive in cleaning agents, a soil conditioner in agriculture, and a popular specimen for mineral collectors and educational kits.

Geological facts

Famous for its property of double refraction (birefringence); when light passes through a clear specimen, it splits into two rays, making objects viewed through it appear double. It was historically used by Vikings for navigation on cloudy days, known as the 'sunstone'.

Field identification & locations

Identify in the field by its rhombohedral shape (like a leaning cube), its softness (can be scratched by a copper penny), and its effervescence (bubbling) when in contact with weak acid. Commonly found in limestone quarries and hydrothermal vein deposits worldwide.

Other mineral specimens

See all

Sandstone (with potential mineral staining/concretions)

Arenite (SiO2 based)

sedimentary

Epidote

Epidote | Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)

metamorphic

Nephrite Jade

Nephrite

Mineral/Rock

Banded Gneiss (River Rock/Pebble)

Gneiss (specifically a banded variety often found as a river rock)

Metamorphic Rock

Green Apatite on Albite

Fluorapatite [Ca5(PO4)3F] on Albite [Na(AlSi3O8)]

mineral

Epidote

Epidote - Ca2(Al2,Fe3+)3(SiO4)3(OH)

mineral