Auctioneers Sotheby's said yesterday that a rare pink diamond valued at up to $US38 million ($39.11 million) would go on sale in Geneva in November, dubbing it one of the "world's most desirable stones". The 24.78 carat Fancy Intense Pink Diamond mounted on a ring comes from a private collection, the auction house said. The gemstone, rated among a type that accounts for just two per cent of diamonds, was last seen on the market about 60 years ago when it was sold by legendary US jeweller Harry Winston.