red coral vs opaque gem

red coral and opaque gem both are nouns.

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

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

red 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

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