Calcite

Hardness: 3 on the Mohs scale. Color: Often white, clear, or colorless, but can be yellow (as seen here), pink, brown, green, or blue due to impurities. Luster: Vitreous to pearly. Crystal structure: Trigonal (rhombohedral is common, but can form in many habits including scalenohedral, prismatic, acicular, or tabular). Cleavage: Perfect rhombohedral cleavage in three directions. Specific Gravity: 2.71. It exhibits double refraction, meaning if you place a clear piece of calcite over text, the text will appear double.

Calcite forms in a wide variety of geological environments. It can precipitate from supersaturated waters, form biogenetically (e.g., in shells and skeletons of marine organisms), or form through metamorphic processes. It is a primary component of limestone and marble. The formation of the crystalline druzy on a host rock, as seen in the image, often occurs from hydrothermal solutions filling cavities or fractures in existing rocks. Geologically, calcite formations range from very ancient (Precambrian) to geologically recent.

Industry: Principal component of cement and lime for agriculture. Construction: Used as a building material (limestone, marble). Chemical Industry: Used as a neutralizing agent. Optical: Clear, high-purity pieces (Iceland Spar) were historically used in polarizing microscopes. Gemstone/Jewelry: While too soft for most jewelry, attractive crystal forms are highly prized by mineral collectors. Carving/Sculpture: Marble, a metamorphic rock composed primarily of calcite, is a famous sculptural material.

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 primary mineral in limestone and marble. Its strong double refraction property was crucial in early optics. It is also the mineral that forms stalactites and stalagmites in caves.

How to identify in the field: Key identification features include its relative softness (easily scratched by a coin, but not a fingernail), perfect rhombohedral cleavage, and strong effervescence (fizzing) when a drop of dilute hydrochloric acid is applied (due to its carbonate composition). The specimen in the image shows a druzy habit, which is a layer of small, sparkling crystals coating a surface, typically on a host rock. Common locations: Calcite is found worldwide in sedimentary, metamorphic, and igneous rocks. Significant deposits are found in Mexico (known for large, clear crystals), Iceland (Iceland Spar), and many cave systems globally. Tips for collectors: Collectable forms include beautiful crystal clusters, fluorescent specimens, and specimens showing interesting habits like dogtooth spar or nailhead spar. Care should be taken with handling as it is relatively soft and cleaves easily.