crown jewel vs jewel

crown jewel and jewel both are nouns.

Word	Noun	Adjective	Verb	Adverb
crown jewel	Yes	No	No	No
jewel	Yes	No	Yes	No

As nouns, jewel is a hypernym of crown jewel; that is, jewel is a word with a broader meaning than crown jewel:

crown jewel: a precious stone that is a valuable part of a sovereign's regalia
jewel: a precious or semiprecious stone incorporated into a piece of jewelry

Other hypernyms of crown jewel include gem, precious stone.

crown jewel (noun)	jewel (noun)
a precious stone that is a valuable part of a sovereign's regalia	a precious or semiprecious stone incorporated into a piece of jewelry
the most desirable assets of a corporation	a person who is as brilliant and precious as a piece of jewelry

crown jewel (verb)	jewel (verb)
	adorn or decorate with precious stones