volcanic rock vs tuff

volcanic rock and tuff both are nouns.

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

As nouns, tuff is a hyponym of volcanic rock; that is, tuff 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
tuff: hard volcanic rock composed of compacted volcanic ash

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

volcanic rock (noun)	tuff (noun)
extrusive igneous rock solidified near or on the surface of the Earth	hard volcanic rock composed of compacted volcanic ash