A flawless fancy vivid pink diamond sold for a record US$17.7 million in Hong Kong late Tuesday, Sotheby's said.

The auction house said the internally flawless 8.41-carat stone had been expected to go under the hammer for up to US$15.5 million.

Quek Chin Yeow, deputy chairman of Sotheby's Asia, said the pear-shaped stone had "attracted keen competition before fetching a wonderful price of HK$137.88 million (US$17.77 million) and setting a world auction record for a fancy vivid pink diamond".

The buyer's identity was not immediately disclosed.

Internally flawless clarity is extremely rare in pink diamonds, and the auction house said this, in combination with its "fancy vivid" colour grading, made the stone "amongst the rarest and most desirable of coloured diamonds ever seen at auction".

Diamonds in the "fancy vivid" category are considered as having the highest possible concentration of colour.