This necklace is, I believe the large sapphire and diamond sautoir and chain which Robert's mentions was ordered by the Queen Mother, when Duchess of York, from A. Sintzenich in 1929.
I believe that this chain could have been made using the stones taken from the large sapphire necklace that Queen Elizabeth received as a wedding-gift from Queen Mary in 1923.
Furthermore the suggestion that is it sapphire, rather than Amethysts lies in the fact that she also wears the Cartier Bandeau, which what is most likely to be the sapphire bracelet in the centre of the two all-diamond bracelets (and there was no amethyst bracelet which could have been worn in conjunction with the amethyst and pearl necklace).
--Previous Message-- : I've been searching everywhere for a pic I : remember seeing - a grainy black-and-white : photograph of the original gift in it's : case. IIRC it looked nothing like the : necklace worn by the DOC. As in the : description from Field, the stones were much : larger and surrounded by brilliants. : : However, there is this photo posted by : Franck recently. Perhaps a longer version : of what Camilla is wearing? : : : : : http://members2.boardhost.com/royal-jewels/msg/1339343985.html : :