crown of thorns vs diadem

crown of thorns and diadem both are nouns.

Word	Noun	Adjective	Verb	Adverb
crown of thorns	Yes	No	No	No
diadem	Yes	No	No	No

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

crown of thorns: a mock crown made of thorn branches that Roman soldiers placed on Jesus before the Crucifixion
diadem: an ornamental jeweled headdress signifying sovereignty

Other hypernyms of crown of thorns include crown.

crown of thorns (noun)	diadem (noun)
a mock crown made of thorn branches that Roman soldiers placed on Jesus before the Crucifixion	an ornamental jeweled headdress signifying sovereignty
somewhat climbing bushy spurge of Madagascar having long woody spiny stems with few leaves and flowers with scarlet bracts
any affliction that causes great suffering