Calcite

Hardness: 2.5-3.5 on Mohs scale; Color: Often white, grey, or off-white, can also be yellow, brown, or pink; Luster: Dull, earthy, powdery, or pearly; Crystal Structure: Rhombohedral, often occurs as massive, fine-grained aggregates; Cleavage: Perfect rhombohedral; Specific Gravity: 2.71 g/cm³

Calcite forms in various geological environments. It can precipitate from calcium-rich waters in caves (forming stalactites and stalagmites), hot springs, and on the ocean floor. It is a common vein mineral in hydrothermal deposits, and a significant component of sedimentary rocks like limestone (formed from the accumulation of shell fragments and marine organisms) and metamorphic rocks like marble (recrystallized limestone). Its formation spans various geological ages, from Precambrian to recent.

Calcite, primarily in the form of limestone and marble, is vital for construction (cement, aggregate, building stone). Pure calcite is used in optical instruments (due to its double refraction), as a flux in metallurgy, in the chemical industry, as a soil conditioner, and as a filler/pigment in plastics, paper, and paints. Small, attractive Calcite specimens are also collected by mineral enthusiasts.

Calcite is one of the most common minerals on Earth, making up a significant portion of the Earth's crust. It is the primary mineral in limestone and marble. Its distinct rhombohedral cleavage and reaction to acid are key identification features. Double refraction is a notable optical property of clear calcite crystals (Iceland Spar), where objects viewed through them appear doubled.

In the field, calcite can be identified by its relative softness (easily scratched by a knife or steel nail), its perfect rhombohedral cleavage (it breaks into rhomb-shaped fragments), and most notably, its effervescence (fizzing) when a drop of dilute hydrochloric acid is placed on it. Common localities include limestone quarries and marble deposits worldwide, cave systems, and hydrothermal vein deposits. Tips for collectors include looking for well-formed crystals in vugs or geodes, and specimens with interesting colors or associations with other minerals.