A dazzling pink diamond, described as one of the world's purest, could fetch more than $21 million when it goes under the hammer in Hong Kong in October, Sotheby's auction house said on Wednesday.

"[Pink diamonds] are exceptionally rare in nature...," Kristian Spofforth, Sotheby's head of jewelry, told Reuters at a press preview.

At 11.15 carats, the cushion-shaped gem is called “Williamson Pink Star,” in tribute to two other pink diamonds.

One is the "CTF Pink Star," a 59.60-carat oval mixed-cut diamond, which sold for a record $71.2 million at auction in 2017. The other is the “Williamson” stone – a 23.60-carat diamond given to Queen Elizabeth as a wedding gift by Canadian geologist John Thorburn Williamson. Worn by the monarch in a Cartier brooch, it was discovered in his mine in Tanzania.

"You then add in the extra factors, like it being over 10 carats, internally flawless and Type IIA, and you get right down to the pinnacle," Spofforth added, referring to a subgroup of the most chemically pure diamonds.

Top-quality coloured stones are prized by the super-wealthy and Spofforth said he expected many bids for the gem when it is offered for sale in a standalone auction on Oct 5.

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