gem vs precious stone

gem and precious stone both are nouns.

Word	Noun	Adjective	Verb	Adverb
gem	Yes	No	No	No
precious stone	Yes	No	No	No

As nouns, gem and precious stone are synonyms defined as:

gem and precious stone: a precious or semiprecious stone incorporated into a piece of jewelry

Other synonyms of gem include jewel.

gem (noun)	precious stone (noun)
a precious or semiprecious stone incorporated into a piece of jewelry	a precious or semiprecious stone incorporated into a piece of jewelry
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