From Longman Dictionary of Contemporary Englishdiademdi‧a‧dem /ˈdaɪədem/ noun [countable] literary ALDCa circle of jewels that a queen, princess etc wears on her head
Examples from the Corpus
diadem• Andronikos also found a diadem, a corselet, helmet and spears, a tripod and metal bowls.Origin diadem (1300-1400) Old French Greek diadein “to tie around” 