low density vs rarity

low density and rarity both are nouns.

Word	Noun	Adjective	Verb	Adverb
low density	Yes	No	No	No
rarity	Yes	No	No	No

As nouns, low density and rarity are synonyms defined as:

Other synonyms of low density include tenuity.

low density (noun)	rarity (noun)
a rarified quality	something unusual -- perhaps worthy of collecting
	a rarified quality
	noteworthy scarcity