headstone vs keystone

headstone and keystone both are nouns.

Word	Noun	Adjective	Verb	Adverb
headstone	Yes	No	No	No
keystone	Yes	No	No	No

As nouns, headstone and keystone are synonyms defined as:

headstone and keystone: the central building block at the top of an arch or vault

Other synonyms of headstone include key.

headstone (noun)	keystone (noun)
the central building block at the top of an arch or vault	the central building block at the top of an arch or vault
a stone that is used to mark a grave	a central cohesive source of support and stability