Weighing 5.5 carats and cut into a striking triangular shape, the gem is the largest blue-green diamond ever recorded. It was originally extracted from an 11.70-carat rough stone unearthed at a mine in Central Africa during the 1990s.

The Smithsonian Institution has classified it as one of the world’s eight rarest diamonds, and it once featured in the museum’s 2003 “Splendor of Diamonds” exhibition in Washington alongside other legendary stones such as the Moussaieff Red and the 203-carat De Beers Millennium Star, Daily Sabah reports.

Max Fawcett, global head of Christie’s Jewellery, described the sale as truly exceptional. “It’s very rare to find green diamonds, even over one carat,” he said. “To find something in 5 carat of this quality and this colour is truly remarkable.”

Christie’s added that the stone is also classified as type Ia, placing it among the purest natural gems known.

Despite its extraordinary rarity, Mr Fawcett noted that the diamond is no bigger than a fingernail. Three bidders from different parts of the world competed for the prize on Wednesday, with the anonymous winning buyer outpacing the gem’s previous 2014 sale price of $8.5 million (c. £6.4m) – more than doubling its value in just over a decade.