One of world's purest pink diamonds is expected to sell for $21 million at auction

LONDON - A rare, dazzling pink diamond is expected to sell for an estimated $21 million when it goes up for auction in October, officials said. 

The 11.15-carat cushion-shaped stone, called the "Williamson Pink Star," was described by auction house Sotheby’s as "one of the world’s purest, most saturated pink diamonds." 

The diamond was named after two other notable pink diamonds, including the record-breaking CTF Pink Star, a 59.60-carat oval mixed-cut diamond, which sold in 2017 for $71.2 million. 

The Williamson Pink Star was also named in homage to the celebrated "Williamson" stone, a cut diamond of 23.60-carats given as a wedding present to Queen Elizabeth II in 1947. It was gifted to the queen by Canadian geologist and royalist, Dr. John Thorburn Williamson, who owned the Williamson mine in Mwadui, Tanzania, where it was discovered.

The Williamson Pink Star also originates from that mine in Tanzania, Sotheby’s said.

A model holds the Williamson Pink Star in a provided image. (Credit: Sotheby’s)

"The discovery of a gem-quality pink diamond of any size is an extremely rare occurrence - something that, with the recent closure of the Argyle mine, seemed until recently - highly improbable," Wenhao Yu, chairman of jewelry and watches at Sotheby’s Asia, said in a statement.

"Driven by a limited supply and rising demand, prices for top-quality large pink diamonds over 5 carats have increased exponentially over the past decade, serendipitously setting the scene for the appearance now of this one-of-a-kind stone," Yu added. 

The Williamson Pink Star is pictured in a provided image. (Credit: Sotheby’s)

What exactly makes the Williamson Pink Star diamond so rare?

Pink is one of the rarest colors to occur naturally in diamonds, according to the auction house. 

"With its unique color, considerable size, and dazzling brilliant cut, the Williamson Pink Star is both a work of nature and a work of art, establishing its place among the most fascinating colored diamonds," the Gemological Institute of America (GIA) said in a statement. The GIA is a nonprofit cited by the auction house as a leading source of knowledge and standards on diamonds.

The Williamson Pink Star is pictured in a provided image. (Credit: Sotheby’s)

Of all the diamonds submitted to the institute, less than 3% are classified as colored diamonds. And of those, less than 5% are considered predominantly pink, according to Sotheby’s. Narrowing in more, only the tiniest proportion of pink diamonds have a strong enough color to be classified as "Fancy Vivid Pink," which the Williamson Pink Star is classified. 

"A Fancy Vivid Pink diamond, weighing over 10 carats is therefore of the utmost rarity. Not only has the diamond received the highest colour and clarity grades, it has also been found to be part of the rare subgroup comprising less than 2% of all gem diamonds - known as Type IIa. Stones in this group are chemically the purest of all," Sotheby’s said. 

The Williamson Pink Star was unveiled on Wednesday at Sotheby’s London and will embark on a world tour, going to Dubai, Singapore, and Taipei before arriving in Hong Kong, where it will be sold in a single-lot auction on Oct. 5. 

This story was reported from Cincinnati.