sardonyx vs onyx

sardonyx and onyx both are nouns.

Word	Noun	Adjective	Verb	Adverb
sardonyx	Yes	No	No	No
onyx	Yes	No	No	No

As nouns, onyx is a hypernym of sardonyx; that is, onyx is a word with a broader meaning than sardonyx:

sardonyx: an onyx characterized by parallel layers of sard and a different colored mineral
onyx: a chalcedony with alternating black and white bands; used in making cameos

sardonyx (noun)	onyx (noun)
an onyx characterized by parallel layers of sard and a different colored mineral	a chalcedony with alternating black and white bands; used in making cameos