rock salt vs common salt

rock salt and common salt both are nouns.

Word	Noun	Adjective	Verb	Adverb
rock salt	Yes	No	No	No
common salt	Yes	No	No	No

As nouns, common salt is a hypernym of rock salt; that is, common salt is a word with a broader meaning than rock salt:

rock salt: naturally occurring crystalline sodium chloride
common salt: a white crystalline solid consisting mainly of sodium chloride (NaCl)

Other hypernyms of rock salt include mineral, sodium chloride.

rock salt (noun)	common salt (noun)
naturally occurring crystalline sodium chloride	white crystalline form of especially sodium chloride used to season and preserve food
	a white crystalline solid consisting mainly of sodium chloride (NaCl)