The De Beers Blue diamond has been auctioned for HK$450.9-million ($57.5-million) at Sotheby’s in Hong Kong. The 15.1 ct, fancy vivid blue diamond was sold Wednesday evening to an unnamed buyer. The step-cut gem, recovered from the Cullinan mine in South Africa, is the largest vivid blue diamond ever to appear at an auction. Bidders had pushed the price to HK$390-million, while auction premiums added the rest.