(IDEX Online) - An exceptional 39.34-carat blue rough diamond could ultimately fetch $64m as a polished gem.The Type IIb diamond (pictured), recovered at the Cullinan mine, South Africa, is set to yield over $1m per rough carat when it's sold at special tender next month by Petra Diamonds.Comparisons with another diamond from the same mine, The Blue Moon of Josephine, suggest it could fetch as much as $64m as a polished gem, says industry veteran Howard Cohen, editor of The Jeweler Blog.The Blue Moon started life as a 29.6-carat rough recovered in 2014. It was bought by New York-based Cora International for $25.6m, transformed into a 12.03-carat fancy vivid internally flawless cushion-cut stone, and sold at Sotheby's to Hong Kong billionaire Joseph Lau for $48.5m.If the newer, larger stone, loses the same proportion in cutting - 59 per cent - it will yield a polished gem of at least 16-carats.At the same price per carat of around $4m, that could translate to a price tag of $64m.The rough stone is being shown in Antwerp, Dubai, Hong Kong and New York and bidding closes on 12 July.