Cave pearl
A cave pearl is a small, usually spherical, speleothem (cave formation) found in limestone caves. Cave pearls are formed by a concretion of calcium salts that form concentric layers around a nucleus. Exposure to moving water polishes the surface of cave pearls, making them glossy; if exposed to the air, cave pearls can degrade and appear rough.
Composition
A cave pearl is composed primarily of calcite. Cave pearls are generally not considered to be a type of oolite. Other minerals found in small quantities in cave pearls include quartz, apatite, iron, aluminum, and magnesium.
Formation
Cave pearls form when water dripping into a cave loses carbon dioxide and precipitates calcite. A cave pearl forms when the water is moving too vigorously to form a stalagmite. A nucleus of matter (such as a grain of sand) becomes coated with calcite, and the current then provides a rotation to the nucleus in such a way that it is coated evenly. In this manner, concentric layers build up over time, in much the same way that a biological pearl forms within a mollusc. There may be microbial action involved in the formation of cave pearls.
