Calcite

Hardness: 3 on the Mohs scale; Color: Variable, often white, colorless, gray, yellow, orange, blue, pink, brown, or black. The specimen appears light yellow/orange; Luster: Vitreous to pearly; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral cleavage (three directions at 74° and 106°), which is often visible as parallel planes or steps; Specific Gravity: 2.71

Calcite forms in a wide variety of geological settings. It is a primary component of sedimentary rocks like limestone and marble (metamorphosed limestone). It precipitates from both marine and fresh waters, often as a biogenic component (shells of marine organisms). It can also form in hydrothermal veins, stalactites, and stalagmites in caves, and as a cementing agent in sandstones. Its formation can occur over vast geological timescales, from Precambrian to recent epochs.

Calcite is widely used in construction (as a primary component of cement and aggregate), agriculture (soil conditioner), optical instruments (Iceland spar variety), and as a flux in metallurgy. It is also used in the chemical industry, as a filler in paper and plastics, and as an ornamental stone. While not typically a gemstone, some attractive varieties are used for carvings or collection.

Calcite is one of the most common and widespread minerals on Earth, making up about 4% by weight of the Earth's crust. It is the principal constituent of pearls and the shells of marine organisms. The 'Iceland spar' variety of calcite exhibits strong double refraction, where light passing through it is split into two rays, causing objects viewed through it to appear duplicated.

Calcite can be identified in the field by its relatively low hardness (easily scratched by a knife or steel nail, but not by a fingernail), its perfect rhombohedral cleavage which can create distinctive blocky or stepped fractures, and its reaction to acid (it effervesces vigorously with dilute hydrochloric acid). It is found globally in sedimentary, metamorphic, and igneous environments. Common locations include limestone quarries, caves, and hydrothermal vein deposits. For collectors, well-formed crystals and interesting color varieties are sought after.