Image: A huge pilar in Postojnska Jama, Slovenia.

speleothem: A secondary mineral deposited in a cave by the action of water. Also known as cave formation.
From: Sybil P. Parker ed. (1997): McGraw-Hill Dictionary of Geology and Mineralogy, 380 pages, McGraw-Hill; ISBN: 0070524327 Bookamazon.com

The most common speleothems consist of Explaincalcite (CaCO3), Explainaragonite (CaCO3) or Explaingypsum (CaSO4). Some common types are: