tufa

from WordNet (r) 3.0 (2006)

tufa
    n 1: a soft porous rock consisting of calcium carbonate
         deposited from springs rich in lime [syn: {tufa}, {calc-
         tufa}]
    2: hard volcanic rock composed of compacted volcanic ash [syn:
       {tuff}, {tufa}]

from The Collaborative International Dictionary of English v.0.48

Tufa \Tu"fa\, [It. fufo soft, sandy stone, L. tofus, tophus. Cf.
   {Tofus}, {Toph}, and {Tophin}.] (Min.)
   (a) A soft or porous stone formed by depositions from water,
       usually calcareous; -- called also {calcareous tufa}.
   (b) A friable volcanic rock or conglomerate, formed of
       consolidated cinders, or scoria.
       [1913 Webster]