chalcedony vs onyx

chalcedony and onyx both are nouns.

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

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

chalcedony: a milky or greyish translucent to transparent quartz
onyx: a chalcedony with alternating black and white bands; used in making cameos

Other hyponyms of chalcedony include agate, bloodstone, heliotrope, carnelian, cornelian, chrysoprase, plasma, sard, sardine, sardius.

chalcedony (noun)	onyx (noun)
a milky or greyish translucent to transparent quartz	a chalcedony with alternating black and white bands; used in making cameos