A cave pearl is a type of speleothem, a concretion of calcium salts which forms in limestone caves. They may be any shape but are formed of concentric layers, reaching up to 15 centimeters (6 in) in diameter. Most are smaller than 1 centimeter (0 in) wide. They can be found singly or grouped. A cave pearl forms as calcite crystallizes on a nucleus such as a grain of sand, in much the same way that a biological pearl forms within a mollusc. A cave pearl is formed when a current of water provides a rotation to the nucleus in such a way that it is coated evenly. In this manner it may form a sphere or a cylinder or any of several other shapes. The cave pearl then sinks and is buffed to a high gloss by the motion of the water. It sinks to the bottom of the pool, and while the motion of the water often keeps it from adhering, several cave pearls may stick together to form stone which looks like a bunch of grapes. Cave pearls often degrade when allowed to dry. There may be microbial action involved in the formation of cave pearls. Cave pearls are generally not considered to be a type of oolite.