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]

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]