opal vs opaque gem

opal and opaque gem both are nouns.

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

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

opal: a translucent mineral consisting of hydrated silica of variable color; some varieties are used as gemstones
opaque gem: a gemstone that is opaque

Other hypernyms of opal include mineral.

opal (noun)	opaque gem (noun)
a translucent mineral consisting of hydrated silica of variable color; some varieties are used as gemstones	a gemstone that is opaque