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Its expected price: $70 million. Lesedi la Rona is an 1109-carat diamond. It's the size of a tennis ball.The gigantic diamond will be auctioned by Sotheby's in London next month. The diamond was found in Botswana in southern Africa in November. It took months to determine even an estimated price, because it is so large that it does not fit into conventional scanners used to evaluate a stone's potential worth.Sotheby's said the diamond is the largest rough, gem-quality diamond in existence today. It is believed to be between 2.5 billion and 3 billion years old.