The largest internally flawless fancy vivid blue diamond ever put up for auction could sell for up to $50 million at a Christie's sale of rare jewels in Geneva on November 7, according to the auctioneer.

Known as 'Bleu Royal,' this vivid blue diamond - which is set inside a ring - is among the rarest ever to be unearthed. 

"What makes Bleu Royal so rare and special is its size. At 17.6 carats, it's the largest of its kind," said Rahul Kadakia, Christie's International Head of Jewellery.

Kadakia said he hopes it will beat the 14.6-carat blue diamond known as the 'Oppenheimer Blue' that Christie's sold in 2016 for $57 million.

The auctioneer will also be presenting a Rolex wristwatch worn by Marlon Brando in the 1979 movie Apocalypse Now, on the back of which the actor engraved his signature to avoid having it swapped accidentally during shooting.

"What makes it truly unique is that it is really one of the most recognizable Rolex memorabilia that has ever surfaced at auction," said Remi Guillemin, Head of Europe Watches at Christie's.

Two years ago the watch, now valued at between 1 million and 2 million Swiss francs, sold at auction for $2 million Swiss francs.

In a separate online auction running between November 3 and 16, Christie's will be offering a pearl necklace worn by film and fashion icon Audrey Hepburn in the final scene of the 1953 movie Roman Holiday. The necklace was created by the renowned Italian jeweler Furst, who catered to well-heeled clients including Hollywood stars.

"What's special about the necklace is that she wore it in the last scene of Roman Holiday, which was such a great movie, which audiences have enjoyed so much, and where she played a young royal enjoying herself in Rome," said Kadakia.

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