I agree with Franck. It is definitely no surprise that the large sapphire has been returned to the Queen's collection. The information suggesting that this was part of the divorce settlement between the Prince of Wales and Diana, Princess of Wales has been circulating since just prior to the official divorce.
For example in The Times 13 July 1996.
Of course the most significant clue that jewels with a royal provenance were returned to the Queen was given by Hugh Roberts in The Queen's Diamonds where the Lover's Knot tiara featured among the Queen's jewels. The fact that the Duchess of Cornwall was seen wearing the Welsh Prince of Wales feathers brooch/pendant reinforced that.
The Times article also stated that gifts received during state visits would also be returned.
What the writer of the article didn't cover was what would happen to gifts given to Diana, Princess of Wales on personal occasions.
As for the origin of the large sapphire many (including myself) have speculated that it might have been one of Mrs Greville's jewels.
Franck spotted that Queen Elizabeth the Queen Mother had worn the jewel in 1953.
I then identified two earlier occasions in the 1940s.
For information on both things see https://mb.boardhost.com/historyroyaljewels/msg/1531775494.html
At the christening of Prince Richard of Gloucester in 1944.