I believe this brooch can be identified as one mentioned in Field on page 147, here are the notes that I have on it:
The Queen’s Cartier Sapphire Floral Spray Brooch
The origins of this brooch are unknown, it is described by Field “a Cartier floral spray set with sapphires, rubies, diamonds and an aquamarine”, this brooch was been presented to The Queen, when still Princess Elizabeth, as a gift from her parents in 1943.[1]
Although this brooch has never been identified pictorially, it is possible that it is the floral spray brooch that The Queen has been pictured wearing on occasion, most notably to the reception hosted at Marlborough House, by the Commonwealth Secretariat to mark The Queen’s Diamond Jubilee in 2012.
Notes: 1] Leslie Field, The Queen’s Jewels, (London: Abradale Press, 1987), p. 147
As it was worn in 2012:
--Previous Message-- : : Joye - I had a go and this the best I could : make of it. : Sorry, but I don't recognise the brooch. : : : : : --Previous Message-- : : Taken from recent Daily Mail article about : Prince William of Gloucester. : : Which brooch is the then Princess Elizabeth : wearing. Sorry for bad photo. My printer is : a lemon. : : : : : :