The exceptionally rare Bleu Royal gem is set to become one of the most expensive diamonds ever sold by auctioneers Christie's when it goes under the hammer on Tuesday.

The Bleu Royal is the star lot in a series of sales in Geneva this month which feature items from movie history including pearls worn by Audrey Hepburn and Marlon Brando's self-engraved watch.

Weighing 17.61 carats, the Bleu Royal is the largest internally flawless fancy vivid blue gem ever to appear for sale in auction history.

The stone is expected to fetch $35 million to $50 million in Christie's Magnificent Jewels auction. 

"It's going to be one of the top 10 jewels sold at Christie's ever, in terms of value," Rahul Kadakia, Christie's international head of jewellery, told AFP.

The diamond, set in a ring, has been in a private collection for 50 years.

Kadakia said lots of modern colored diamonds had a modified cut to enhance the color.

In the Bleu Royal's case, "the rough material itself was so rich that they were able to cut it into a classic brilliant faceting style," he said.

"It checks all the boxes — really top of its class."

Fancy vivid blue diamonds weighing more than 10 carats are exceptionally rare. Since Christie's was founded in 1766, only three such stones have appeared for sale — all in the last 13 years.

The Bulgari Blue sold for nearly $15.8 million in 2010; the Winston Blue fetched $23.8 million in 2014, and the Oppenheimer Blue realized $57.5 million in 2016.

"This level of the jewellery market has turned into the same sort of appreciation as art," said Kadakia.

"The market saw there's a great scarcity of these special stones and they started making prices in the same manner as great pictures do."

– Movie memorabilia –

The separate online jewels sale, which runs until November 16, features the pearl necklace worn by Hepburn in the 1953 film "Roman Holiday".

Hepburn, who played a princess in the romantic comedy, picked it out from a selection offered by the Austro-Hungarian jeweller Furst, and returned it afterwards with a signed photograph.

It is being sold by a European collector and has a starting price of 18,000 Swiss francs ($20,000), but could go much higher in a bidding war.

"You only need two people to make a party," said Max Fawcett, the head of jewellery at Christie's in Geneva.

"Audrey Hepburn is an iconic figure. Her fashion, clothes and anything that she ever wore is still highly sought after by collectors," he told AFP.

"People love a story and private clients want to be part of those stories."

– 'Apocalypse Now' watch –

In Monday's Passion for Time watches sale, the timepiece worn by Brando in the 1979 Vietnam War movie "Apocalypse Now" is estimated to fetch one to two million Swiss francs.

The circa 1972 Rolex GMT-Master is hand-engraved "M. Brando" on the back by the Oscar-winning US actor himself.

"The engraving is still sharp, so we can imagine that this watch has been worn, but not too much," said Eli Fayon, junior watches specialist at Christie's Geneva.

Told that the eye-catching watch would distract moviegoers, Brando removed the bezel — the outer ring which encircles the crystal glass to keep it in place.

Besides the engraving, "that is also what makes this watch unique", Fayon told AFP.

Brando gifted it to his adopted daughter Petra in 1995, who then gave it to her husband in 2003 on their wedding night. It was sold at auction in 2019 for $1.95 million.

Its white hands and hour markers have aged to parchment yellow.

"Putting aside that it's Marlon Brando's watch, the watch in itself is amazing. The condition is spectacular for a 50-year-old watch," said Fayon.

He was expecting interest from vintage Rolex collectors and clients that want "not only a piece of watch history but also movie history". (Robin MILLARD)

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© Agence France-Presse