A 9.51-carat Fancy Vivid Blue diamond, known as The Mellon Blue, is set to headline an upcoming auction in Geneva this November. Graded Internally Flawless and mounted in a ring, the stone will be offered during Christie’s Magnificent Jewels sale at the Four Seasons Hotel des Bergues on November 11, 2025. The diamond will be on public display in Hong Kong from October 23 to 28 before the sale. It will then be exhibited in Geneva from November 7 to 11 at the Four Seasons Hotel des Bergues, ahead of the auction.

The diamond carries an estimated value of $20 to $30 million USD. Its history traces back to Rachel Lambert Mellon—widely known as Bunny Mellon (1910–2014)—an American horticulturist, philanthropist, and art collector. Mellon acquired the gem decades ago when it was set as a pendant. The piece last appeared publicly in 2014, when it was sold in New York and ranked among the highest-priced coloured diamonds ever auctioned.

Beyond her jewellery collection, Mellon left a lasting mark through her work in landscape design. In 1961, President John F. Kennedy invited her to redesign the White House Rose Garden, where she incorporated American plant species and created a more open layout for public events. In France, she collaborated with designer Hubert de Givenchy on the gardens of his residence and contributed to the restoration of the “Potager du Roi” at the Château de Versailles.

The appearance of The Mellon Blue at auction reconnects the diamond with its notable provenance, drawing attention to both its rarity and its association with a figure whose influence extended from art and design to political history.