Cave pearls are not actually pearls, they are a calcareous concretion occurring in subterranean limestone caves. While they bear a resemblance to pearls, they have a porous texture. Cave pearls form when calcite crystallizes on a nucleus, similar to the formation of a pearl. Water flowing over the nucleus rotates it for relatively even coverage and, ultimately, polishes the result. Their porosity makes most examples too fragile for use in jewelry but they do occasionally turn up set as jewels.