gem vs gemstone

gem and gemstone both are nouns.

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

As nouns, gem and gemstone are synonyms defined as:

gem and gemstone: a crystalline rock that can be cut and polished for jewelry

Other synonyms of gem include stone.

gem (noun)	gemstone (noun)
a precious or semiprecious stone incorporated into a piece of jewelry	a crystalline rock that can be cut and polished for 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