volcanic rock vs tufa

volcanic rock and tufa both are nouns.

Word	Noun	Adjective	Verb	Adverb
volcanic rock	Yes	No	No	No
tufa	Yes	No	No	No

As nouns, tufa is a hyponym of volcanic rock; that is, tufa is a word with a more specific, narrower meaning than volcanic rock:

volcanic rock: extrusive igneous rock solidified near or on the surface of the Earth
tufa: hard volcanic rock composed of compacted volcanic ash

Other hyponyms of volcanic rock include agglomerate, amygdaloid, dacite, basalt, volcanic glass, lava, tuff.

volcanic rock (noun)	tufa (noun)
extrusive igneous rock solidified near or on the surface of the Earth	a soft porous rock consisting of calcium carbonate deposited from springs rich in lime
	hard volcanic rock composed of compacted volcanic ash