The Mellon Blue, a 9.51-carat fancy vivid blue diamond formerly owned by the US art collector Bunny Mellon, sold for US$25.6 million at auction on Tuesday.

The diamond was sold by auctioneers Christie's in Geneva as part of its Magnificent Jewels sale in the Swiss city.

The gem sold for 20,525,000 Swiss francs at the Hotel des Bergues, having been estimated to fetch between 16 and 24 million francs.

Rahul Kadakia, who conducted the sale, said the price was evidence of the "elite appetite amongst collectors for extraordinary and storied gems."

The stone, set in a serpentine ring, once belonged to Rachel Lambert Mellon, better known as Bunny Mellon, a US horticulturalist, philanthropist and art collector.

"The Mellon Blue is an outstanding stone," said Max Fawcett, head of jewellery at Christie's in Geneva.

"More than that, though, it has also become symbolic of a lifestyle of a bygone era – the type only possible with a Gilded Age fortune."

The brilliant-cut stone is graded as internally flawless.

The diamond was last seen publicly in 2014, when it was auctioned in New York after her death at the age of 103, selling for more than US$32 million.

Meanwhile, Sotheby's is holding its annual Royal and Noble Jewels sale in Geneva on Wednesday.

A brooch seized from Napoleon Bonaparte as he fled the Battle of Waterloo in 1815, laden with old mine-cut diamonds has been valued at US$150,000 to US$250,000.

Also going under the hammer is a 10.08-carat vivid pink diamond, named "The Glowing Rose", which is expected to rake in around US$20 million.

The gem "of exceptional quality" was named after its "extremely rare" luminous and pure pink colour, Sotheby's said. (AFP)