I'm afraid I have little photographic evidence for this, but, I was at the archives in Windsor castle for some research, and was sitting opposite a painting of Queen Alexandra, when princess of Wales, and she is wearing a brooch very similar to the one in this thread - a round pearl surrounded by diamonds, and with a large drop [I think again diamonds]. Obviously this is not the Cambridge Pearl as that came through Queen Mary, not is it the wedding gift brooch from her husband, which we saw the Queen wear in Northern Ireland recently, as Alexandra is also wearing that brooch lower down her bodice. I'm sorry that I don't have a photo of this, because it is not allowed to take phones or cameras into the royal archives. I will look and see if I can find another image of this brooch.