A magnificent blue diamond has fetched $US32.6 million ($A35.27 million) in New York, breaking the world auction record for any diamond of its colour, auction house Sotheby’s says.

The stunning 9.75 carat, pear-shaped diamond was bought by a Hong Kong private collector after 20 minutes of competitive bidding on Thursday evening, Sotheby’s said on Friday.

The price also set a new world auction record for price-per-carat for any diamond, the auctioneers said.

The jewel had been owned by Rachel Lambert otherwise known as Bunny Mellon, the art collector wife of the late philanthropist and racehorse breeder Paul Mellon. She died in March aged 103.

“Mrs Mellon’s diamond absolutely deserves the place in the record books that it achieved,” said Gary Schuler, head of Sotheby’s jewellery department in New York.

It was the highlight of a collection of Mellon’s jewels and other prized objects that Sotheby’s sold for more than $US42 million in New York on Thursday, well above the pre-sale estimate of $US19.2 million.