The BBC has an interesting article on the largest unseen diamond:
The Wittelsbach-Graff Diamond was last seen in public at the 1958 World Exhibition in Brussels. After that, it disappeared and its whereabouts remained a mystery until Laurence Graff, a billionaire diamond dealer, bought it at auction in 2008, appending his surname.
He and his son Francois were in the gallery of the Smithsonian’s National Museum of Natural History in Washington DC as the blue stone, was brought up from a secret vault and finally revealed.
Some great pictures in the article too: