Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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