Travertine is a kind of limestone deposited by springs. Groundwater traveling through limestone beds dissolves calcium carbonate, an environmentally sensitive process that depends on a delicate balance between temperature, water chemistry and carbon dioxide levels in the air. As the mineral-saturated water encounters surface conditions, this dissolved matter precipitates in thin layers of calcite or aragonite, two crystallographically different forms of calcium carbonate (CaCO3). With time, the minerals build up into deposits of travertine.