A cave pearl is a type of speleothem that forms in moving or agitated water, one of a range of secondary mineral formations created by the precipitation of minerals from water in caves - usually calcium carbonate (calcite or aragonite – CaCO3), or calcium sulphate (gypsum – CaSO4 2H2O). As water drips into pools (or trickles across a surface), layers of carbonate precipitate out and accumulate around a nucleus (such as a sand grain).
This process is very similar to the way that pearls form in shelled molluscs, whereby the mollusc secretes layers of carbonate-containing mucus around the ‘irritant’, which hardens to form the pearl. Almost all types of shelled molluscs have the ability to form pearls, however it is the pearls from pearl oysters and mussels that are prized as valuable gems. Curiously, research has indicated that biogenic mucous (biofilm) from microbes may play an important role in the formation of cave pearls which makes them even more similar to their molluscan counterparts.
During their formation, cave pearls are agitated or even rolled around in the water, allowing them to remain detached from the bed. It’s also what leads to their polished, smooth appearance. However, when the pearls get too large and are no longer able to be moved by water, they become cemented to the bed. This is well illustrated in the photograph where the nest of smaller, loose pearls is surrounded by larger, cemented ones.
Interestingly, speleothems such as cave pearls could be used to infer details of past climate change by the measuring oxygen isotope ratios that were locked up in during their formation. However, their limitations as a proxy are still being investigated.
The Baatara Gorge Waterfall (Balaa Gorge Waterfall)
It is a waterfall in the Tannourine, Lebanon. The waterfall drops 255 metres (837 ft) into the Baatara Pothole, a cave of Jurassic limestone located on the Lebanon Mountain Trail. Discovered in 1952 by French bio-speleologist Henri Coiffait, the waterfall and accompanying sinkhole were fully mapped in the 1980s by the Speleo club du liban. The cave is also known as the “Cave of the Three Bridges.”