A spectacular and rare orange diamond, the largest known gem of its kind, has sold for a record $US31.5 million ($A33.74 million) in Geneva.

“At the back of the hall, 29 million francs. Sold!” the Christie’s auctioneer said as the fiery almond-shaped gem was snapped up in a room of about 200 people in a luxury Geneva hotel.

The price excluded another $US4.04 million in taxes and commission.

The man who made the purchase swiftly got up and left the room to a round of applause.

Christie’s did not reveal his identity.

The deep orange gemstone, which was found in South Africa, weighs a whopping 14.82 carats. The Gemological Institute of America (GIA) has handed it the top rating for coloured diamonds: “fancy vivid”.

Pure orange diamonds, also known as “fire diamonds”, are extremely uncommon and very few have been auctioned, with the largest never more than six carats.

“To have one that’s over 14 carats is exceptional,” Christie’s international jewellery director David Warren told AFP.

He said “The Orange” was “the largest recorded vivid orange diamond in the world”.

In 1990 the 4.77-carat yellow-orange Graff Orange diamond was sold for $US3.92 million dollars and in 1997 the vivid orange Pumpkin diamond of 5.54 carats was sold for $US1.32 million.

Christie’s had estimated “The Orange” would rake in $US17 million to $US20 million.

Coloured diamonds, once considered a curiosity, are rarer than white diamonds and today attract higher prices per carat than even the most flawless, translucent stone.

That, Warren explained, was because, “coloured diamonds are real freaks of nature. They begin as white diamonds, and it’s some accidental colouring agent in the ground that will turn it a particular colour”.