earthstory:

Cave Pearls

These concretions vary from spherical to almond or pill shape and form like natural pearls around a nucleus, but in caves rather than oysters or mussels. The water in limestone caves is usually saturated with calcium carbonate from the limestone through which it has percolated into the cave, which gets precipitated around the nucleus. The pearls acquire a high polish and become more spherical if they are rolled in the moving waters in which they form, eventually being dropped into potholes and other cavities where they form these birds nest formations. The calcium carbonate can precipitate as either calcite or aragonite.

Loz

Image credit: 1: WTucker 2: NPS 3: Kevin Zim