tombstone vs monument

tombstone and monument both are nouns.

Word	Noun	Adjective	Verb	Adverb
tombstone	Yes	No	No	No
monument	Yes	No	No	No

As nouns, monument is a hypernym of tombstone; that is, monument is a word with a broader meaning than tombstone:

Other hypernyms of tombstone include memorial, stone.

tombstone (noun)	monument (noun)
a stone that is used to mark a grave	a structure erected to commemorate persons or events
	a burial vault (usually for some famous person)
	an important site that is marked and preserved as public property