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: Webster's Revised Unabridged Dictionary (1913)