rareness vs rare

rareness is a noun but rare is not a noun.

Word	Noun	Adjective	Verb	Adverb
rareness	Yes	No	No	No
rare	No	Yes	No	No

rareness (noun)	rare (noun)
noteworthy scarcity

rareness (adjective)	rare (adjective)
	marked by an uncommon quality; especially superlative or extreme of its kind
	recurring only at long intervals
	(of meat) cooked a short time; still red inside
	not widely distributed
	having low density
	not widely known; especially valued for its uncommonness