\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.