Pink diamond expected to fetch more than £20m at Hong Kong auction

The second largest, internally flawless “fancy vivid pink” diamond ever to appear at auction is expected to sell for more than £20m when it goes under the hammer at Sotheby’s in Hong Kong on Friday.

The 11.15-carat, cushion-shaped diamond, known as the Williamson Pink Star, is named after two other huge pink diamonds: the 59.60-carat, mixed-cut, oval Pink Star diamond that sold for a record $71.2m at auction in 2017, and the Williamson stone, a 23.60-carat diamond given to the late Queen Elizabeth II as a wedding gift by the Canadian geologist and ardent royalist John Thorburn Williamson in 1947.

Mounted as a floral brooch designed by Frederick Mew of Cartier in 1953, the Williamson is said to have been a favourite of the Queen who wore it on many occasions during her reign, including the silver jubilee.

Williamson owned the Mwadui mine in Tanzania where the Williamson stone and the Pink Star were discovered.

Wenhao Yu, the chair 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.”

Argyle, a Rio Tinto-owned diamond mine in the remote north of Western Australia, was closed in 2020 after 37 years of operation, during which it produced more than 865m carats of rough diamonds.

“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,” Yu said.

Pink diamonds are particularly rare among coloured diamonds and no one quite knows exactly how they become pink geologically.

“While nitrogen and boron are responsible for the vivid hues of yellow and blue diamonds, respectively, there is no evidence that pink diamonds receive their colour from trace elements,” Sotheby’s said.

“Rather, the crystal structure of the stone selectively absorbs light as a result of an idiosyncratic lattice defect which results in an unusual arrangement of atoms in the crystal. These happy anomalies occasionally cause pink graining in the diamond crystal – a perfectly brilliant display of imperfection.”