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Cave formations are called speleothem, from the Greek word spelaion, cave and thema meaning deposit. Almost all of them are made of calcite, the crystal of calcium carbonate. Different minerals and different movements in the water account for an incredible variety of cave formations.
This water can dissolve limestone over time, creating a cave. The same water that dissolved the calcite in the limestone can deposit the calcite in other areas of the cave, creating cave formations known as speleothems. These are the stalactites, stalagmites and other formations commonly found in this type of cave.