We don't have synonyms for iron crown yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are iron cross and ron brown.
(n) :
(Christianity) A reliquary from the early Middle Ages, consisting of a circlet of gold and jewels fitted around a central silver band, which tradition held to be made of iron beaten out of a nail of the True Cross.
(n) :
(Christianity) A reliquary from the early Middle Ages, consisting of a circlet of gold and jewels fitted around a central silver band, which tradition held to be made of iron beaten out of a nail of the True Cross.