"(Pink diamonds) are exceptionally rare in nature...," Kristian Spofforth, Sotheby's head of jewellery, told Reuters at a press preview.
"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," he 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.
"We saw over the lockdown crisis and the COVID crisis that there is always demand for the rare and the beautiful in the world, and this is something exceptionally rare that I think will have plenty of bidders on the day," he said.
Ahead of the auction, the "Williamson Pink Star" will go on tour to Dubai, Singapore and Taipei before arriving in Hong Kong.