tu·fa 
(tfə, ty-)
(tfə, ty-)n.
A soft, friable, and porous sedimentary rock consisting of calcium carbonate and formed by the evaporation of water, especially at the mouth of a hot spring or on a drying lakebed. It can also be precipitated by algae or bacteria.
[Obsolete Italian tufa, tufo, from Latin tōfus.]
tu·faceous (-fāshəs) adj.