If the Queen is indeed wearing (as it looks like now having this new photograph) the Greville Honeycomb tiara for this event, then I do not see why it could not also be the large Greville Diamond necklace around the Queen's neck.

This does indeed raise a lot of questions though: was the jewels borrowed from Mrs Greville? Had Mrs Greville already given her jewels to the Queen. Were some of Mrs Greville's jewels gifted to the Queen during her lifetime while the rest only past to the Queen after Mrs Greville's death? And if they were borrowed, why did not the Queen borrow from, say, Queen Mary instead of asking a private citizen to lend her some of her grandest jewels? It is not as if the Queen was without access to grand jewels as I believe she had already by that time been given Queen Mary's Fringe tiara. And of course, the double Greek meander tiara was lingering in the vaults..

It is all very intriguing and confusing at the same time.