grindstone vs sharpener

grindstone and sharpener both are nouns.

Word	Noun	Adjective	Verb	Adverb
grindstone	Yes	No	No	No
sharpener	Yes	No	No	No

As nouns, sharpener is a hypernym of grindstone; that is, sharpener is a word with a broader meaning than grindstone:

grindstone: a revolving stone shaped like a disk; used to grind or sharpen or polish edge tools
sharpener: any implement that is used to make something (an edge or a point) sharper

Other hypernyms of grindstone include stone.

grindstone (noun)	sharpener (noun)
a revolving stone shaped like a disk; used to grind or sharpen or polish edge tools	any implement that is used to make something (an edge or a point) sharper