calcareous tufa[kal′ker·ē·əs ′tü·fə]
or travertine, porous, cellular rock formed from the precipitation of calcium carbonate from hot or cold springs. It often contains impressions of plants and sometimes the shells of land or freshwater gastropods. It is characterized by a small density (1,400–1,800 kg/m3). In the USSR calcareous tufa commonly occurs near Piatigorsk, near Yerevan, in Podolia, and near Pudozh. It is used as a building material, as decorative stone, and for roasting lime.