showpiece vs rarity

showpiece and rarity both are nouns.

Word	Noun	Adjective	Verb	Adverb
showpiece	Yes	No	No	No
rarity	Yes	No	No	No

As nouns, rarity is a hypernym of showpiece; that is, rarity is a word with a broader meaning than showpiece:

showpiece: the outstanding item (the prize piece or main exhibit) in a collection
rarity: something unusual -- perhaps worthy of collecting

Other hypernyms of showpiece include curio, curiosity, oddity, oddment, peculiarity.

showpiece (noun)	rarity (noun)
the outstanding item (the prize piece or main exhibit) in a collection	something unusual -- perhaps worthy of collecting
	a rarified quality
	noteworthy scarcity