By Jane O'Brien
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.
"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.
Under ultraviolet light, the diamond has an orange hue
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.
"When I saw this stone, I knew it was a stone we had to have," said Laurence Graff. "I had the opportunity to examine and value it in my own offices, and I came to the conclusion it was one of the rarest stones I'd ever seen."
Initially valued at around $15m, Mr Graff paid more than $25m. In a controversial move he had it re-cut and polished, reducing it from 35.5 carats to little over 31 carats. Critics say the act compromised the historical integrity of the stone, but Mr Graff disagrees.
"I decided that to create beauty, or acts of beauty, is not a sin. All we did was remove the blemishes and now it's true perfection. It's the most wonderful diamond to hold in your hand. It's got the most incredible feel to it - a magical feel. We have managed to bring out the true colour of the stone without changing the faceting or the shape.
Laurence Graff paid more than $25 million for the gem
"The true rarity of the diamond, whatever its history, is the diamond itself. The history will continue. Every diamond that was ever mined, every diamond that was ever polished and cut, is still with us. Who knows the story that this stone will tell in a thousand years time. I'm sure it will gather very romantic stories, mysterious stories, intriguing stories, but at the end of the day the true beauty of the stone will remain. That will always be the best story."
Francois Graff compared the decision to restoring a priceless painting. "If you discovered a Leonardo da Vinci with a tear in it and covered in mud, you would want to repair it. We have similarly cleaned up the diamond and repaired damage caused over the years."
The Wittelsbach-Graff Diamond originated in India. In 1664, Philip IV of Spain gave it to his daughter, the Infanta Margarita Teresa to celebrate her engagement to Emperor Leopold of Austria.
In 1772, it acquired its name by passing to the Wittelsbach family of the House of Bavaria. After World War I, Bavaria became a republic and the Crown Jewels, including the diamond were sold.
Then it was rarely seen in public.
"It's probably the most famous diamond the world has never seen," said Jeffrey Post, curator of the Smithsonian National Gem Collection. "It's always been lurking out there - but we've never seen it."
The Wittelsbach-Graff was secretly transported to the museum in the dead of night, a week before it was due to go on display.
The Hope and the Wittelsbach are believed to be in a class by themselves
Mr Post and some of the nation's leading diamond experts locked themselves in the vault to spend the time examining it and comparing it to the legendary Hope - the world's largest blue diamond at 45.52 carats - which is the star of the National Gem Collection.
It was thought that the two may once have been part of the same crystal, but tests proved negative. Although they share significant similarities and come from the same place, the Hope and the Wittelsbach-Graff are more like distant cousins than siblings.
"We had this confluence of history with two of the world's great diamonds," said Mr Post, "and an unparalleled opportunity to examine them. Diamonds have been mined for hundreds of years but in all that time these two stand in a class by themselves unlike any other diamond found."
Diamonds are typically formed about 100 miles underground and are billions of years old. They can tell scientists much about the history of the planet.
The Wittelsbach-Graff will be displayed alongside the Hope at the Smithsonian's National Museum of Natural History until August. After that, its future is uncertain.
Laurence Graff says it may move to London's Natural History Museum - but it could also be sold. If that happens, it could disappear for another hundred years.