That's not the Cullinan. It wasn't dug out of the ground until 1905! I'll look it up for you: there's another brooch that looks like the parts you're thinking of.

Queen Alexandra is wearing the purple robe. The page nearest is holding up Edward VII's parliament robe but the Queen Consort also wear her robe throughout the day and you can just make it out (also held up, by a page in the doorway) if you look carefully.