RAPAPORT... A 10.64-carat pink diamond ring worth up to $25.5 millionwill go under the hammer at Sotheby's Hong Kong sale. The fancy-vivid-purplish-pink center stone is internallyflawless, the auction house told Rapaport News Friday. Sotheby's willoffer the piece, which is flanked by white diamonds, at its Magnificent Jewelsand Jadeite sale on October 7. The ring has a presale estimate of HKD 150million to HKD 200 million ($19.1 million to $25.5 million). Pink diamonds have historically done well at auction, withthe Pink Promise, a 14.93-carat, fancy-vivid pink, fetching HKD 250 million($32 million) at Christie's Hong Kong sale in 2017. In November, Harry Winstonpurchased the Pink Legacy, an 18.96-carat, fancy-vivid-pink diamond, atChristie's Geneva auction for $50.4 million. Prices of pink fancy-color diamonds increased 0.4% in the three months ending June 30, outperforming blues and yellows, the Fancy Color Research Foundation noted. The imminent closure of Rio Tinto's Argyle mine in Australia has contributed to the price rise, as the deposit produces most of the world's pinks, explained Alan Bronstein, president of the Natural Color Diamond Association.Image: The 10.64-carat pink diamond. (Sotheby's)