Cave Pearls form when water dripping into a pool loses carbon dioxide and precipitates calcite around a nucleus (a grain of sand, or fragments of soda straws). The typical roundness is due to the uniform growth of the pearl, not to any sort of rotation due to dripping…they can be square or irregular pearl also. 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. Cave pearls may variate in size (this ones are quite as big as marbles, but sometimes they can can reach the size of a golf ball) and in color, it depends on minerals dissolved in water. Once the the pearls are removed by their nest or water in the pool dries out, they are bound to turn to dust again.