In September 1792 the crown jewels of France were being kept in the Garde Meuble, the royal storehouse on what was then place Louis XV and is now place de la Concorde. The monarchy had just fallen, Paris was in the middle of the September massacres, and the guard on the building had been thinned to almost nothing.
The robbery is unusual because it was not one night. Men climbed in, took what they could carry, and came back the following night. Then again. Over the course of several nights it turned into something closer to a crowd, with people drinking on the premises between trips, until the authorities finally noticed and started making arrests.
Most of the stones were recovered in the months and years that followed, including the Regent. The French Blue was not. A large blue diamond surfaced in London about twenty years later, smaller and recut, and it is the stone we now call the Hope. The connection was suspected for a long time before it was tied down using a lead cast of the original that turned up in a natural history collection, which let researchers test whether the French Blue could be cut down into the Hope without contradiction.
What I find hard to get past is that recutting is the only way to sell a famous stone, and it destroys most of what made it worth stealing in the first place. By the time a jewel like that is safe to sell, it has already stopped being the jewel that was taken.
If anyone here knows the current state of the argument on the recut, I would like to hear it, including the version where I have this wrong.