The largest oval vivid fancy diamond ever to appear is headed for sale in Hong Kong next month, where it is expected to fetch $30 million to $35 million. The internally flawless blue diamond, at 10.10 carats, was mined in South Africa and is one of a collection of 11 cut by jewellers De Beers in London in 2010. The blue diamond will lead an April 5 Sotheby’s auction in Hong Kong after being exhibited in London, New York, Geneva, Singapore and Taipei.