»carcanet

carcanet (kär'kə-nĕt', -nĭt): "A jeweled necklace, collar, or headband."

Perhaps from Medieval Latin carcannum, which means "pillory" or "prison"; ARTFL gives a more complete etymology: [Dim. fr. F. carcan the iron collar or chain of a criminal, a chain of precious stones, LL. carcannum, fr. Armor. kerchen bosom, neck, kechen collar, fr. kelch circle; or Icel. kverk troat, OHG, querca throat.] A jeweled chain, necklace, or collar. [Also written carkenet and carcant.].

Some dictionaries mark this as archaic.