If you had one job, and that job was to safeguard the Crown Jewels from the Nazis, where (and how) would you hide the priceless gems?
The gems had to be shielded from various threats.
The first priority was to transfer them from London to a safer location, and Windsor Castle got the nod.
Even outside London, the gems faced several threats. A bomb could blast them to smithereens.
If the Wehrmacht successfully invaded Britain (the German Operation Sea Lion was designed to do just that), Nazi search parties might get their hands on them—and the invaders would have looked specifically for the jewels.
The task of safeguarding the treasures, which included historical documents and works of art by Leonardo da Vinci and Holbein, fell to the King’s librarian, Sir Owen Morshead. His orders came directly from King George VI.
An underground chamber bolstered by steel doors was erected at Windsor Castle.
The crowns had been shipped from the Tower in ordinary hat boxes, but Morshead had a better idea. He pried the precious stones out of the crowns and placed them in a glass jar which he then inserted into an ordinary biscuit tin. The tin did not draw attention to itself, and in an emergency, it could easily be moved.
Morshead also inserted a note “signed by the King to say that it had been done at his personal direction, and that it was at his wish that nobody had been told” (Shenton, p.193).
After the war, the gems were reinserted into the crown and returned to the Tower of London.
Caroline Shenton, National Treasures: Saving the Nation’s Art in World War II (2021)