The fancy vivid blue-green diamond is triangular in shape and “the size of the nail on your smallest finger”, according to Fawcett.

It was sold for 13.6m Swiss francs (US$17.3m) in Geneva, a new record for a blue-green diamond at auction, it said.

“We sold the stone in 2014 for eight and a half million dollars. It was bought by a private Asian collector who enjoyed it. She wore it,” Fawcett said.

The auction saw three clients from different parts of the world bidding on the gem. The winner had chosen to remain anonymous, Fawcett said.

The gem was first extracted from a rough stone weighing 11.70-carats, Christie’s said.

It was cut and exhibited at the Smithsonian Institute’s Museum of Natural History in Washington, DC, in 2003 as part of a Splendour of Diamonds exhibit.

The United States show featured red, orange, yellow, pink, blue, blue-green and white diamonds, ranging from 5.11-carats of the Moussaieff Red to 203.04-carats of the De Beers Millennium Star.

– AFP