Laos

In Xe Bang Fai, a river cave in Laos, a grain of sand can become the seed of a pearl.

Dripping water keeps the grain rolling while calcium carbonate builds around it, layer by layer. The movement helps the pearl grow and stay glossy and rounded. Once it grows too heavy to keep moving, the pearl can fuse to the cave floor. 

That’s why most cave pearls stay small, rarely bigger than a walnut. The very largest, though, are too big to close a hand around. 

Unlike ordinary pearls, where sand is rarely the real cause despite the myth, a cave pearl often starts with exactly that: a grain of sand.