An “exceptionally rare” pink diamond, named the Williamson Pink Star in a nod to a similar precious stone owned by the Queen, is expected to sell for more than 21 million dollars (£18 million) this autumn.

The 11.15 carat cushion-shaped stone is the second largest internally flawless fancy vivid pink diamond to ever appear at auction and could set a new per carat price record at auction, Sotheby’s said.

It is outsized only by the CTF Pink Star which sold in 2017 for 71.2 million US dollars (£57.2 million), setting a world record for any diamond or jewel, according to the auction house.

Pure pink diamond sale
Model Nina Uhl wearing a large, pure pink, diamond on display at Sotheby’s, central London, before it is offered at auction this autumn (Yui Mok/PA)

The Williamson Pink Star is named after its record-breaking predecessor, as well as the Queen’s Williamson pink diamond – which was discovered at the same mine in Tanzania.

The 23.6 carat diamond was a wedding gift to the Queen in 1947 from Canadian geologist Dr John Thoburn Williamson, who owned the mine, and became the centrepiece of a brooch designed by Cartier.

Sotheby’s head of jewellery Kristian Spofforth told the PA news agency: “The name is really nice because it references one of the most famous pink diamonds in the world which is in the Cartier brooch worn by the Queen.

“It came from the same mine that this diamond comes from which is the Williamson mine in Tanzania, so there is a lovely link there.

“We don’t know a huge amount of detail about the Queen’s diamond because it is very private and given to the Queen in 1947.

Pure pink diamond sale
The last time a gem of this colour, quality and size was shown to the world was in 2017 and it went on to become the most expensive gemstone ever to be sold at auction (Yui Mok/PA)

“Then you also have the link to the Pink star which was sold in 2017, which set the world record price for any gemstone or jewel at auction, and the fact they were both cut by Diacore brings this wonderful link into it which has been tied into the name.”

Mr Spofforth described the stone as “one of earth’s rarest natural beauties”.

He said: “It is exceptionally rare to get a diamond of this colour and purity. We have called it the purest, pinkest diamond you can get.

“Only 5% of diamonds are coloured diamonds; of coloured diamonds, only 3% are pink. You add on the fact of being over 10 carats, internally flawless, and you get into this exceptional rarity that brings us this diamond.

“The vendor is exceptionally excited about bringing this to auction because it is such a natural beauty.”

He added that the diamond had been in “exceptional hands” since it came out the ground as a rough a few years ago, adding that the transformation to polished diamond takes “a while”.

The Gemological Institute of America (GIA) monograph said: “With its unique colour, 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 coloured diamonds.”

Wenhao Yu, chairman of jewellery and watches at Sotheby’s Asia, said: “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.

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

The diamond is expected to fetch in excess of 21 million dollars during the Sotheby’s auction on October 5 in Hong Kong.