Calcite

Hardness: 3 (Mohs scale), can be scratched by a copper coin. Color: White, colorless, gray, yellow, green, pink, brown, or black. The specimen in the image appears to be white to very light pink/purple. Luster: Vitreous to pearly on cleavage surfaces. Crystal Structure: Hexagonal (rhombohedral). Cleavage: Perfect rhombohedral cleavage in three directions, producing characteristic parallelepiped fragments. Specific Gravity: 2.71.

Calcite forms in a wide variety of geological environments. It can precipitate from hot springs (travertine), crystallize from magmas (rarely), be a primary component of sedimentary rocks (limestone, chalk), or form as a secondary mineral in veins and cavities in igneous and metamorphic rocks. It is also the primary mineral in marble, a metamorphic rock. Its formation often involves the presence of water and dissolved calcium and carbonate ions. The geological age varies widely depending on the specific formation.

Calcite is used in construction as a primary component of cement and as aggregate. In agriculture, it's used as a soil conditioner (limestone). Optically clear calcite (Iceland Spar) is used in polarizing microscopes and other optical instruments. It is also used as a filler in paper, paints, and plastics. While not a primary gemstone, some colorful or well-formed specimens are collected and occasionally used in decorative carvings or cabochons. It is also an important ore for calcium.

Calcite is one of the most common minerals on Earth, constituting about 4% by weight of the Earth's crust. It is the principal constituent of limestone and marble. It exhibits strong birefringence, meaning that light passing through it is split into two rays. This is most noticeable in clear, transparent varieties like Iceland Spar, where objects viewed through it appear doubled.

Calcite can be identified in the field by its relatively low hardness (can be scratched with a knife, but not a fingernail), its perfect rhombohedral cleavage, and its reaction to acid (it fizzes vigorously when a drop of dilute hydrochloric acid is applied, producing carbon dioxide gas). It is found globally in almost every country, often in sedimentary deposits, metamorphic terrains, and hydrothermal veins. Good specimens are commonly found in limestone quarries, caves, and mineral veins in various rock types.