I think that Queen Mary is wearing and all diamond necklace. The darkness if just light refraction. I cannot make out the brooch.
It is interesting to note how formal of an event this was back in the day. Queen Elizabeth is wearing her Royal Family Orders and her Garter Star with day dress for a luncheon. It is very rare to see such formality after the war. I cannot think of too many time when QEII has worn orders with a day dress.