Tufa - Calcium carbonate (in the form of Calcite or Aragonite, depending on crystallization) that forms in caverns from precipitating water while trapping in organic matter, such as leaves, twigs, and moss. Also calcareous mounds formed from deposition of hot springs that traps in organic matter. Layers may be banded. Synonym of Calcareous Tufa, and Calcareous Sinter, and Sinter. See also Travertine.