BBC News, Washington
The room was dimly lit. Armed guards stood at both entrances and enormous ironclad doors were slid shut to seal the gallery.
Nobody spoke above a whisper as we waited for the first glimpse in half a century of one of the world's most extraordinary gems.
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.
Flawless beauty
"I've been privileged over the years to own some of the world's most important and famous diamonds, but I would say that the Wittelsbach-Graff is the most valuable and the most 
beautiful," he said.
Sitting unadorned on a silken white cloth, it glittered grey and blue in the low light. Classed as "internally flawless" it is said to have exceptional colour and becomes intense orange when viewed under ultraviolet light.
