Thank you for posting such a really interesting image, The Royal Watcher. It raises all sorts of issues.
1. Do you know when and why the tiara was displayed?
2. Why do you think that Queen Victoria's sapphire tiara was the last one owned by the family?
But my biggest query is --
Do others think that the tiara (as per The Royal Watcher's image) and the spray brooch worn by Marion Stein, Countess of Harewood are the same jewel?
I don't think so.
Comparison
Marion Stein wearing the spray jewels in 1956.
Detail
From a newspaper item, a slightly different view.
And in 1957. Same dress, different opera and different year.