Calcite

Hardness: 3-4 (Mohs scale); White to colorless, often with tints of yellow, brown, pink, or gray; Vitreous to pearly luster; Orthorhombic crystal system, commonly forming tabular or prismatic crystals, often twinned; Perfect cleavage in one direction, good in two others; Specific gravity: 2.8-3.0.

Calcite forms in a wide range of geological environments. It can precipitate from hot or cold aqueous solutions, often as veins or crusts in igneous and metamorphic rocks. It also forms extensively through biological processes in marine environments, where organisms extract calcium and carbonate ions to build shells and skeletons. Significant biogenic calcite accumulation leads to the formation of limestone. It can also form during metamorphism of carbonate-rich rocks. Geological age varies greatly, from Precambrian to modern times, depending on the specific formation process.

Industrial uses include an essential component in cement and concrete manufacturing, as a flux in steel production, as a soil conditioner in agriculture, and as a filler and pigment in paints, plastics, and paper. In construction, limestone (composed primarily of calcite) is a major building material. High-purity calcite (optical calcite) is used in optical instruments such as polarizers and microscopes. While not generally used in jewelry due to its softness, some attractive specimens are collected.

Also known as Iceland Spar when found in large, transparent cleavage rhombohedrons, exhibiting strong double refraction. This property makes it historically significant for optical instruments, polarizing filters, and in early experiments on light. Calcite is one of the most common minerals on Earth and is a primary component of many sedimentary and metamorphic rocks. It is also the main constituent of shells of marine organisms and cave formations (stalactites and stalagmites).

Calcite can be identified in the field by its relatively low hardness (easily scratched by a knife or steel nail), its perfect rhombohedral cleavage (producing distinctive blocky fragments when broken), and its effervescence (fizzing) when a dilute acid (like vinegar or hydrochloric acid) is applied. It's very common and found worldwide in sedimentary rocks (limestone, chalk), metamorphic rocks (marble), and as a vein mineral in igneous rocks, as well as in caves (speleothems). To distinguish it from similar-looking minerals like quartz, remember its lower hardness and reactivity to acid.