gem vs opaque gem

gem and opaque gem both are nouns.

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

As nouns, opaque gem is a hyponym of gem; that is, opaque gem is a word with a more specific, narrower meaning than gem:

Other hyponyms of gem include cabochon, transparent gem, semi-precious stone.

gem (noun)	opaque gem (noun)
a precious or semiprecious stone incorporated into a piece of jewelry	a gemstone that is opaque
art highly prized for its beauty or perfection
a sweet quick bread baked in a cup-shaped pan
a person who is as brilliant and precious as a piece of jewelry
a crystalline rock that can be cut and polished for jewelry