(IDEX Online) - The largest blue diamond ever to be auctioned today sold for just under $57.5m at Sotheby's Hong Kong - way above its $48m estimate, but just short of the Oppenheimer Blue's $58m record.The Type IIb 15.1-carat, IF fancy vivid step cut Cullinan Blue (pictured) went to an unnamed buyer.The hammer price was HK$390m, plus HK$60.9m in auction premiums, totaling HK$450.9m - $57.5m at today's exchange rate. The price per carat was $3,805,561.In May 2016, Christie's Geneva sold the 14.62-carat VVS1 Oppenheimer Blue for CHF 56,837,000, equivalent at prevailing rates to $58m.The Cullinan Blue was recovered in April 2021 by UK-based Petra at the Cullinan mine, South Africa, as a 39.34-carat rough gem, which was sold to a De Beers/Diacore partnership for $40.18m.It is the largest internally flawless step cut vivid blue diamond ever graded by the GIA (Gemological Institute of America).