Calcite

Hardness: 3 on Mohs scale. Color: Typically white, colorless, gray, but can be yellow, pink, blue, brown, or black due to impurities. Luster: Vitreous to pearly on cleavage surfaces. Crystal Structure: Trigonal, commonly in rhombohedral crystals, scalenohedrons, or tabular forms; also massive, granular, fibrous, or stalactitic. Cleavage: Perfect rhombohedral in three directions. Specific Gravity: 2.71. It exhibits double refraction.

Calcite forms in a wide variety of geological settings. It is a primary mineral in sedimentary rocks like limestone and marble (metamorphic limestone). It can precipitate from hot or cold springs (forming travertine or tufa), as a cementing agent in sandstones, or as a secondary mineral in igneous rocks. It's also biogenic, forming shells and skeletons of many marine organisms. Its geological age spans from ancient Precambrian to modern times.

Industry: Principal component of cement and lime, used as a flux in steel making, as an abrasive, and in the production of glass. Construction: Limestone and marble are widely used as building materials. Jewelry/Collectibles: Transparent, colorful, or well-formed crystals are sought after by collectors. Optical grade calcite (Iceland Spar) is used in polarizing microscopes and other optical instruments due to its strong birefringence.

Calcite is one of the most common minerals on Earth, constituting about 4% by weight of the Earth's crust. It is a crucial component of stalactites and stalagmites in caves. It reacts vigorously with dilute acids (like hydrochloric acid), producing effervescence (fizzing) due to the release of carbon dioxide gas. The variety known as 'Iceland Spar' is famous for its exceptionally clear cleavage rhomboids and strong double refraction.

To identify calcite in the field, look for its relatively low hardness (easily scratched by a knife or steel nail), its distinctive perfect rhombohedral cleavage, and its reaction to acid. It often forms translucent to opaque masses or clear crystals. It is found globally in sedimentary environments (limestone, chalk), metamorphic rocks (marble), and as vein fillings in various rock types. Collectors often look for well-formed crystals, particularly in cave formations or mineral veins.