Since the little Prince's first BD party was strictly private, I fear we'll never see a picture of HM - or at least not one, where a brooch would be perfectly visible... On DM's pictures (who - in opposition to their written words - are often the best availabe), one can only spot a little glimpse of the upper part of HM's brooch. IMO, a cluster of diamonds, set in yellow gold... Given the colour of The Queen's dress and the number & size of the diamonds, I guess it could be the Albert Sapphire Brooch (the brownish touch may be due the light).