A 9.51-carat fancy vivid blue diamond, known as the ‘Mellon Blue’, sold for 20.5 million Swiss francs ($26.6 million) at Christie’s auction in Geneva on Tuesday (11 November), including fees. The pear-shaped gem had been estimated to sell between $20 million and $30 million.

Named after the late American arts patron Rachel “Bunny” Mellon, the diamond had been part of her private collection for decades.

Christie’s notes that unlike many contemporary gems, the stone’s natural facets and vivid colour have been preserved, earning it a grading of Fancy Vivid Blue and Internally Flawless from the Gemological Institute of America.

Previously, the Mellon Blue had sold at auction in 2014 for $32.6 million, shortly after Mellon’s death, making it one of the most expensive coloured diamonds ever sold.

Speaking about the sale, Tobias Kormind, managing director of online jeweller 77 Diamonds, said the auction “was not the dazzling moment I expected.”

He cited geopolitical tensions, global market uncertainty, and a softer Chinese luxury market as factors that may have tempered bidding.

Christie’s auctioneer Rahul Kadakia, Chairman of the global luxury group at Christie’s, described the result as a “notable moment,” highlighting the enduring demand among collectors for rare and historic gemstones.

The auction marked the opening of a two-day jewellery series in Geneva. On Wednesday, rival auction house Sotheby’s is scheduled to offer the “Glowing Rose” pink diamond, anticipated to reach around $20 million.

While far below the record for a vivid blue diamond — the 14.62-carat Oppenheimer Blue, which sold for more than $57 million in Geneva in 2016 — the Mellon Blue remains one of the most celebrated coloured diamonds on the market today.

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