A blue diamond weighing nearly 10 carats has sold at auction in Switzerland for 20.5 million Swiss francs ($26.6 million) including fees.

The 9.51-carat “Mellon Blue” — named for the late American arts patron Rachel “Bunny” Mellon — had been expected to fetch $20-$30 million at the Christie's auction on Tuesday.

The house said the stone came in on the estimated range. The final price includes the “buyer's premium” and other fees.

The diamond was previously sold in 2014, the year that Mellon died, for $32.6 million, which was one of the highest prices ever paid for a colored diamond at auction, Christie's says.

For decades, the stone was part of Mellon's private collection.

Max Fawcett, Christie’s global head of jewelry, said the Mellon Blue was unlike the vast majority of other modern gems that have had facets added and been modified to enhance the color.

“When you have great shape and great color, you’re looking at the gem of gems,” he said Friday, noting the stone's grade of Fancy Vivid Blue and Internally Flawless by the Gemological Institute of America. “That's what this is.”

The auction was the first installment of two days of jewelry auctions in Geneva. On Wednesday, rival Sotheby's is putting up the “Glowing Rose” pink diamond that's expected to draw bids of around $20 million.