transparent gem vs gemstone

transparent gem and gemstone both are nouns.

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

As nouns, gemstone is a hypernym of transparent gem; that is, gemstone is a word with a broader meaning than transparent gem:

transparent gem: a gemstone having the property of transmitting light without serious diffusion
gemstone: a crystalline rock that can be cut and polished for jewelry

Other hypernyms of transparent gem include gem, stone.

transparent gem (noun)	gemstone (noun)
a gemstone having the property of transmitting light without serious diffusion	a crystalline rock that can be cut and polished for jewelry