That is the all diamond 'The Queen's Vanguard Rose Brooch'; which she received when launching the ship The Vanguard. The ship that later took the Royal Family to South Africa.
It is all diamonds, and as far as I am aware, still with The Queen. Although it has not been seen in many years.
--Previous Message-- : : Is this the brooch in the QD? : : : : : --Previous Message-- : The picture from page 113 of Queen Mary's : jewel book has an interesting brooch (?) on : the bottom row. Second from the left it : looks like a little diamond oak leaf with a : pearly for an acorn. : Could this be the one that Kate wore on the : pink dress to the Jubilee lunch at Windsor : recently. After all and oak leaf is (now) : her families emblem ? : :