That is the famed Buccleuch belt which can also be worn as a tiara.The Duchess (grandmother of the current Duke) also wears the famous emerald necklace and earrings to which there also is a matching tiara.
--Previous Message-- : Lovely portrait of Duchess Mary Buccleuch, by : her sister-in-law, Molly Bishop (Lady George : Scott), wearing a very impressive tiara. : Does anyone know if this is one of the five : tiaras she had to chose from at the : Coronation of Queen Elizabeth II? : : http://www.boughtonhouse.org.uk/htm/tour/duchessmarypick.htm :