precious coral vs opaque gem

precious coral and opaque gem both are nouns.

Word	Noun	Adjective	Verb	Adverb
precious coral	Yes	No	No	No
opaque gem	Yes	No	No	No

As nouns, opaque gem is a hypernym of precious coral; that is, opaque gem is a word with a broader meaning than precious coral:

precious coral: the hard stony skeleton of a Mediterranean coral that has a delicate red or pink color and is used for jewelry
opaque gem: a gemstone that is opaque

precious coral (noun)	opaque gem (noun)
the hard stony skeleton of a Mediterranean coral that has a delicate red or pink color and is used for jewelry	a gemstone that is opaque