diadem vs crown jewels

diadem and crown jewels both are nouns.

Word	Noun	Adjective	Verb	Adverb
diadem	Yes	No	No	No
crown jewels	Yes	No	No	No

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

diadem: an ornamental jeweled headdress signifying sovereignty
crown jewels: regalia (jewelry and other paraphernalia) worn by a sovereign on state occasions

Other hypernyms of diadem include jeweled headdress, jewelled headdress.

diadem (noun)	crown jewels (noun)
an ornamental jeweled headdress signifying sovereignty	regalia (jewelry and other paraphernalia) worn by a sovereign on state occasions