Well, there's a history, going back to Queen Victoria, of queens leaving some items of jewellery to the Crown rather than as personal property. Among other things, Queen Victoria left the Indian tiara (the diamond and ruby tiara that the Queen Mother wore) to the Crown, as well as the diamond collet necklace that the Queen wore to her coronation and the two strings of pearls that Princess Elizabeth wore at her wedding. The George IV diadem, which is what the Queen's wearing on all the postage stamps, is another one that was left to the Crown by Queen Victoria, and it's only worn on State occasions. It shouldn't be hard to add the Delhi Durbar tiara to the jewels that are Crown property, if it hasn't already been done.
I'm not sure where the George IV diadem lives when it isn't being used, but the Delhi Durbar tiara could join it. Seems a pity for historic pieces like that to not be viewable by the public on a regular basis.
I'm not sure where the George IV diadem lives when it isn't being used, but the Delhi Durbar tiara could join it. Seems a pity for historic pieces like that to not be viewable by the public on a regular basis.