I am also 100% sure it is Queen Adelaide's diamond brooch.
And I agree with Bryce that it could be the Lahore pendant. On the following picture (from the same photo session), and on this picture on this link to a 2013 post by Nellie, we can see more clearly the caracteristical shape of the Lahore diamond: http://members2.boardhost.com/royal-jewels/msg/archive/1362872276.html
I know that the question has already been raised (and, if I remember, never solved) but I am curious about the whereabouts of the necklace. These tassels are definitely not to my taste, but could they have ended up transformed into other jewels?