(IDEX Online ) - An 11.15-ct pink diamond described as "rare masterpiece of nature" is to be sold in a single-lot auction in Hong Kong with a $21m estimate.The cushion cut Williamson Pink Star (pictured), named after the Petra mine in Tanzania, is the second largest internally flawless fancy vivid pink diamond to ever appear at auction.It was sold last December as a 32.32 carat pink rough gem for $13.8m to multinational manufacturer Diacore. Chairman Nir Livnat said it was a "rare masterpiece of nature".The diamond, unveiled last week at Sotheby's London, will be displayed in Dubai, Singapore, Taipei and Hong Kong, where it will be auctioned on 5 October.The only pink diamond larger than the Williamson Pink Star to be sold at auction was the 59.60-carat CTF Pink Star, sold by Sotheby's for $71.2m in April 2017 to China's biggest jeweler, Chow Tai Fook.It still holds the world auction record for any diamond, gemstone or jewel.