World's most valuable diamond set for auction in Geneva The world's most expensive pink diamond is set to fetch over 60 million US dollars (40 million euros) at auction. According to David Bennett, Chairman of Sotheby's International Jewellery division, the Pink Star diamond was discovered in Africa in 1999 and has never before been up for public sale. Bennett pointed out that the pink colour is not an imperfection, saying: "The reason the diamond is pink is probably by an accident when it was being formed. The cause of the pink is not precisely known. What we do know is it is not an impurity," he said. He also noted that the reason diamonds of any kind are so exciting is that they are formed very deep within the earth's crust, and are mined differently other precious stones. The only reason we have them is when they have been associated with volcanoes when they have been brought up by chance to the surface," he said. He added: "We don't mine diamonds like we mine rubies and emeralds. Diamonds are formed several hundred kilometres below the surface of the earth completely inaccessible. The 59.6 carat flawless gem will go under the hammer in Geneva on November 13.