The polished blue gem was cut from a 29.6-carat diamond discovered last year in South Africa’s Cullinan mine. Blue diamonds are formed when boron is mixed with carbon when the gem is created but even at the Cullinan mine which is the “only reliable source in the world for blue diamonds” only a tiny percentage of those found in it contain even a trace of blue. Prior to auction experts took five months for an “intense study” of the diamond. “For me, the blue moon was always the blue diamond of my career. I have never seen a more beautiful stone. The shape, the color, the purity — it’s a magical stone,” said auctioneer David Bennett Another big diamond called ‘Sweet Josephine’ was sold for $28.5 million at Christie’s in Geneva a day earlier to the same buyer.