New tumbled stones in 2025, pistachio calcite from Mexico.

The mineral calcite (also known as limespar) consists mainly of the salt calcium carbonate (CaCO₃) and is one of the most common minerals in the earth's crust. Calcium carbonate is formed when soluble calcium ions come into contact with CO₂ (carbon dioxide forms a carbonate ion when there is a positive ion in a solution). Calcium carbonate is a poorly soluble salt, which causes the formed CaCO₃ to precipitate; this occurs according to the following reaction: Ca²⁺(aq) + CO₃²⁻(aq) -→ CaCO₃(s) Calcite is a common constituent of sedimentary rocks, veins, and deposits in hot springs and caves in karst areas (as stalactites). It is the mineral that forms limestone and is therefore the most common mineral of biological origin. Calcite is the main constituent of limestone, chalk and marble and is also an important constituent of marl, calcareous sandstone and chalk shales. Calcite is responsible for the chemical weathering of igneous and metamorphic rocks, which results in the formation of sedimentary rocks. Calcite is soluble in CO₂-containing water under slight pressure. As it falls into the air, rainwater absorbs the gaseous carbon dioxide, making it acidic. This acidic rainwater is able to dissolve lime. In underground currents, this material can be transported in solution until, after contact with atmospheric pressure, CO₂ escapes and the calcite precipitates. This is how stalagmites and stalactites are formed in caves. In the past, the transparent Icelandic form of calcite, called Iceland spar, was used to make Nicol prisms. Some researchers assume that the Vikings navigated at sea with the help of crystals of this mineral, which can be used to accurately determine the position of the sun even in cloudy weather. Nowadays, limestone is mainly used in construction as a building element and for the production of lime and cement, while marble is used as a building and ornamental stone.