thank you all for your great feedback and details, yes it is the brooch from "Duchess Clementine" which I use for this picture, but it is the only clear picture. I will note it in detail - your are right Arthur.
Now the links for May Goelet Duchess of Roxburghe The wedding gifts www.royal-magazin.de/england/roxburghe/goelet-may-wedding-marriage-jewels.htm