opal vs black opal

opal and black opal both are nouns.

Word	Noun	Adjective	Verb	Adverb
opal	Yes	No	No	No
black opal	Yes	No	No	No

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

opal: a translucent mineral consisting of hydrated silica of variable color; some varieties are used as gemstones
black opal: a dark colored opal with internal reflections of green or red

Other hyponyms of opal include fire opal, girasol, harlequin opal.

opal (noun)	black opal (noun)
a translucent mineral consisting of hydrated silica of variable color; some varieties are used as gemstones	a dark colored opal with internal reflections of green or red