Mellon Blue Diamond Sells for $25.6 Million at Christie's

A 9.51-carat fancy vivid blue diamond once owned by American philanthropist and art collector Rachel Lambert Mellon sold for 20.5 million Swiss francs, equivalent to about $25.6 million, at a Christie's auction in Geneva on Tuesday.

The gem, known as the Mellon Blue, fetched the price during the auction house's Magnificent Jewels sale at the Hotel des Bergues. It had been estimated to bring between 16 million and 24 million Swiss francs, or roughly $20 million to $30 million. The final hammer price fell within that range, though it included a buyer's premium that pushed the total to approximately $26.6 million. Graded internally flawless by the Gemological Institute of America, the pear-shaped stone is set in a serpentine ring and represents one of the rarest types of colored diamonds, with fancy vivid blues comprising less than 0.1 percent of all diamonds.

The sale marks a notable decline from the diamond's previous auction in 2014, shortly after Mellon's death at age 103, when it commanded $32.6 million at Sotheby's in New York. That transaction set a record at the time for a blue diamond and per-carat price for any diamond, though subsequent sales have surpassed it, including...