diadem vs coronet

diadem and coronet both are nouns.

Word	Noun	Adjective	Verb	Adverb
diadem	Yes	No	No	No
coronet	Yes	No	No	No

As nouns, coronet is a hyponym of diadem; that is, coronet is a word with a more specific, narrower meaning than diadem:

diadem: an ornamental jeweled headdress signifying sovereignty
coronet: a small crown; usually indicates a high rank but below that of sovereign

Other hyponyms of diadem include crown of thorns.

diadem (noun)	coronet (noun)
an ornamental jeweled headdress signifying sovereignty	margin between the skin of the pastern and the horn of the hoof
	a small crown; usually indicates a high rank but below that of sovereign