A plum-size pink diamond is expected to break the world record for a gemstone next week when it goes under the hammer in Hong Kong, Sotheby's auction house said yesterday. The 59.60-carat Pink Star is the largest in its class graded by the Gemological Institute of America. But Sotheby's was forced to buy back the flawless diamond after a previous buyer defaulted on what was then a record sale in 2013. That sale in Geneva saw a New York-based diamond cutter secure the stone for US$83 million. Sotheby's is confident the Pink Star will again sell for a record price next Tuesday, and estimate it will fetch more than US$60 million (S$83.7 million). The sparkling oval-cut rock measures 2.69cm by 2.06cm and weighs 11.92g. Sotheby's deputy chairman for Asia Quek Chin Yeow said Hong Kong is the right place for the sale, with Asian buyers breaking records in other categories for diamond sales in the past few years. The Pink Star was discovered in a mine in Africa by De Beers in 1999.