Cave Pearls

Pearls are a concentric concretion found in shallow cave pools. They can be spherical, as in these photos, or cylindrical, elliptical, and even cubical (as in the additional photos in the table below. They range in size from barely larger than a sand grain up to golf-ball sized. In the tropics, large beds of them may be found. Grutas de Canicas, a cave recently explored in Mexico, contained pearls esitmated in the millions.
Cave pearls form when water dripping into the pool loses carbon dioxide and precipitates calcite. This precipitate usually forms around a nucleus of sand, bones, or fragments of soda straws or rafts. The typical roundness is due to the uniform growth of the pearl, not to any sort of rotation due to dripping. A sphere allows the greatest amount of deposition for the smallest surface area and is thus most likely, even if the nucleus is highly irregular. The dripping causes vibrations in the pool which may prevent the pearls from cementing (with calcite) to the pool floor, though many pearls are found cemented in. Sometimes excess precipitate will form cups or nests around the pearls, like in the photo on the top.