tufa

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.

A rock having a rough or cellular texture, sometimes a fragmental volcanic material, and sometimes a calcareous deposit from springs.

A soft or porous stone formed by depositions from water, usually calcareous; — called also calcareous tufa.

A friable volcanic rock or conglomerate, formed of consolidated cinders, or scoria.

the calcareous deposit of lime found near hot springs

the volcanic rock tuff

hard volcanic rock composed of compacted volcanic ash

a soft porous rock consisting of calcium carbonate deposited from springs rich in lime