Optical Calcite

Physical Properties

Hardness: 3 (Mohs scale), Color: Clear/Colorless with white or yellowish tints, Luster: Vitreous to pearly, Crystal structure: Trigonal/Rhombohedral, Cleavage: Perfect rhombohedral in 3 directions, Specific gravity: 2.71

Formation & Geological History

Formed through chemical precipitation from hydrothermal fluids or in sedimentary environments such as limestone cavities. This specimen shows a rhombohedral cleavage shape typical of Icelandic Spar.

Uses & Applications

Historically used in optical instruments (polarizing prisms, gunsights), educational displays to demonstrate double refraction, and for mineral collecting.

Geological Facts

Also known as 'Iceland Spar', it was famously used by Vikings for navigation (the Sunstone) to find the sun through clouds due to its polarizing properties. It exhibits double refraction (birefringence), meaning objects viewed through it appear as a double image.

Field Identification & Locations

Identify by its distinct rhombohedral shape and the property of double refraction when placed over text. Found globally with famous deposits in Iceland, Mexico, and the USA. Collectors look for high clarity and well-defined cleavage planes.

Identified on: 5/13/2026

Mode: Standard