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Cartier. Ruby and diamond necklace and earrings, 1940-1950 (circa, earrings), 1954 (necklace). The central element of the necklace detaches for wear as a clip brooch. In original box. Maker's marks. 2001,1103.1. © The Trustees of the British Museum

Stiff collars worn tight round the neck were fashionable in the early post-war years. Like many such necklaces, the central element detaches for wear as a clip brooch. The pointed clasp with its fancy-cut diamond is an integral part of the collar. 
Although cased together by Cartier, the earrings are different in style from the necklace so must have been made at a different time and were not designed to match. They are unmarked.