I have other photos of a largish diamond brooch belonging to Princess Marina. I doubt if this brooch is the one described in the official wedding catalogue as the one given by the Duke's siblings and their spouses.
I don't have any information about whether the brooch in the photos below belonged initially to Princess Marina. I am posting the photos only in case some one else has photos of Princess Marina wearing the brooch - perhaps these images might trigger memories.
I think that below is a different brooch again - but I may be mistaken.