In my opinion the necklace in the postage stamp and in the post aren't a match. There is a photo of the Empress on which the stamp seems to be based upon. We can see that the necklace she is wearing is a different design.
I wonder if she is wearing it in the first photo for the following. The shape of the central element seems to conform, but I'm not sure as for the rest.
The necklace was previously auctioned by Christies in 2010. The page shows a full view of the necklace.