What is "cave pearl"

n. A kind of speleothem, formed 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 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.