sharpener vs grindstone

sharpener and grindstone both are nouns.

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

As nouns, grindstone is a hyponym of sharpener; that is, grindstone is a word with a more specific, narrower meaning than sharpener:

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

Other hyponyms of sharpener include pencil sharpener, steel, strickle, strop.

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