It is one of the most common and widespread minerals in nature. It is found in all limestones and is stable at all temperatures and pressures. It is of great scientific importance for being a classic example of a mineral that presents the phenomenon of double refraction. Calcite is a rock-forming mineral found in limestone and most sea shells. This rock former is very common on the Earth's surface. It dissolves in water and grows anywhere water can reach. Shellfish make their shells out of calcite, which they absorb from seawater. Some shells are coated with shiny nacre, from which an oyster makes its pearl. Stalactites hang from the ceilings of limestone caves. They grow when calcite precipitates (forms fine deposits) from dripping water. In the heat and low pressures of the caves, the water evaporates and leaves a small deposit of calcite. Stalagmites grow where water falls to the ground.