semi-precious stone vs gem

semi-precious stone and gem both are nouns.

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

As nouns, gem is a hypernym of semi-precious stone; that is, gem is a word with a broader meaning than semi-precious stone:

Other hypernyms of semi-precious stone include gemstone, stone.

semi-precious stone (noun)	gem (noun)
a gemstone of lesser value.	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