Rare Blue Diamond It weighs 7.03 carats and is one of only a handful of blue diamonds in existence in the world. The diamond was found in South Africa and is on show at Sotheby's, in Mayfair, until auction. It will be auctioned in Geneva . The gem, which was cut from a 26.58 carat rough diamond, was discovered in 2008 at the Cullinan mine. It may set a world record price per carat.