bric-a-brac vs rarity

bric-a-brac and rarity both are nouns.

Word	Noun	Adjective	Verb	Adverb
bric-a-brac	Yes	No	No	No
rarity	Yes	No	No	No

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

Other hypernyms of bric-a-brac include curio, curiosity, oddity, oddment, peculiarity.

bric-a-brac (noun)	rarity (noun)
miscellaneous curios	something unusual -- perhaps worthy of collecting
	a rarified quality
	noteworthy scarcity