The extremely rare and flawless "Blue Moon Diamond" sold for 48.6 million Swiss francs ($48.4 million) to a Hong Kong buyer on Wednesday, setting a world record for a gemstone at auction, Sotheby's said.The cushion-shaped diamond, mounted on a ring, has the top grading of fancy vivid blue and weighs 12.03 carats. It had a pre-sale estimate of $35 million-$55 million."It is a new record price for any gemstone and per carat," David Bennett, worldwide chairman of Sotheby's international jewellery division who conducted the sale in Geneva, told a packed showroom that erupted into applause. ($1 = 1.0043 Swiss francs)