tufa

English Thesaurus

1. hard volcanic rock composed of compacted volcanic ash (noun.substance)

hypernym	:	volcanic rock,
definition	:	extrusive igneous rock solidified near or on the surface of the Earth (noun.substance)

2. a soft porous rock consisting of calcium carbonate deposited from springs rich in lime (noun.substance)

hypernym	:	rock, stone,
definition	:	material consisting of the aggregate of minerals like those making up the Earth's crust (noun.substance)