A pink diamond has been sold for $US49.9 million ($A78 million) in Hong Kong, setting a world record for the highest price per carat for a diamond sold at auction.

The 11.15-carat Williamson Pink Star diamond, auctioned by Sotheby’s Hong Kong, sold for $HK392 million. It was originally estimated at $US21 million.

The Williamson Pink Star draws its name from two legendary pink diamonds.

The first is the 23.60-carat Williamson diamond, which was presented to the late Queen Elizabeth II as a wedding gift in 1947.

The second is the 59.60-carat Pink Star diamond that sold for a record $US71.2 million at auction in 2017.

The Williamson Pink Star is the second-largest pink diamond to appear at auction. Pink diamonds are among the rarest and most valuable of the coloured diamonds.

“This is an astounding result, proving the resilience of top diamonds in a shaky economy,” said Tobias Kormind, managing director of 77 Diamonds.

“Hard assets such as world-class diamonds have a history of performing well even in times of instability.

“Some of the world’s highest-quality diamonds have seen prices double over the last 10 years.”

-AAP