Cave pearls

Did you know - I certainly didn't - that pearls can form in caves much like they form in the shell of an oyster? They are the result of calcite forming around a nucleus, such as a grain of sand, in a limestone cave. If water current or drips rotate the nucleus in such a way that it is coated evenly, a cave pearl up to 15cm in diameter forms. It then sinks in the pool and is buffed to a high gloss by the motion of the water. This motion keeps it from adhering to the cave floor, but the pearls may adhere to each other in a form that looks like a bunch of grapes. In the passage below from the National Geographic website, American Mark Jenkins narrates the exploration of an enormous uncharted passage in the massive Hang Son Doong [“mountain river cave”] on the border between Vietnam and Laos with British team members Gareth “Sweeny” Sewell and Howard Clarke:

"It’s just the three of us now, exploring. No human has ever been here before. We drop down off the backside of the Great Wall and begin ascending a staircase of rock toward the exit. 'Will ye look at deese!' roars Clarky, kneeling beside a dried-up pool. Sweeny and I gather around. Inside the pool, illuminated by our headlamps, are cave pearls. Cave pearls are formed when a drop of water from the ceiling hits the limestone floor and throws up a speck of rock. This grain is jostled in its little cup of stone every time a drop hits it. Over thousands of years, a solid, almost perfectly round calcite pearl is formed. Pearls are rare and in most caves are no larger than a marble. The cave pearls here are the size of baseballs, larger than any the cavers have ever seen. (Their preternatural size may be due to the enormous distance the ceiling waterdrops fall.) 'I ’ereby christen this passage Pearl ’arbor,' Clarky announces."

Cave pearls in Vietnam's Hang Son Doong (1st image), California's Crystal Cave (2nd image), and New Mexico's Carlsbad Caverns (3rd image).