gem vs cabochon

gem and cabochon both are nouns.

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

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

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

gem (noun)	cabochon (noun)
a precious or semiprecious stone incorporated into a piece of jewelry	a highly polished gem that is cut convexly but without facets
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