(IDEX Online) - A rare 6.70-carat VS1 natural fancy deep yellow diamond from the famous Zimmi mine, in Sierra Leone, could fetch as much as $705,000 at auction.Canary yellow diamonds from Zimmi can command double the price of other fancy yellows because of their uniquely high color saturation levels. Yellow Zimmi stones over five carats are extremely rare. This gem (pictured) will lead the Tiancheng International Jewellery and Jadeite Autumn Auction 2020 in Hong Kong on 28 November.Also at the sale is a pair of Zimmi natural fancy deep yellow diamond earrings (3.03-carat and 3.01-carat) with an estimate of up to $320,000.The Zimmi mine is in the heart of the rainforest region of Sierra Leone where the Mano river marks the border with Liberia.Yellow diamonds occur when a single nitrogen atom replaces a carbon atom, absorbing the violet and blue light.