A dense, finely cristalline massive or concretionary limestone color, often having a fibrous or concentric structure and splintery fracture, formed by rapid chemical precipitation of calcium carbonate from solution in surface and ground waters, as by agitation of stream water or by evaporation around the mouth or in the conduit of a spring, esp. a hot spring. It also occurs in limestone caves, where it forms stalactites, stalagmites, and other deposits; and as a vein filling, along faults, and in soil crusts. The spongy or less compact variety is tufa. See also: onyx marble. Syn: calcareous sinter; calc-sinter.