"Ocean Dream," the largest blue-green diamond ever recorded, sold for $17 million on Wednesday, Christie's auction house said.

The 5.5-carat diamond was extracted from a mine in Central Africa in the 1990s and has been named by the Smithsonian Institution as one of the world's eight rarest diamonds, Christie's said in a statement announcing the sale.

"A stone of this color and size is extremely scarce and adding to its rarity the diamond is type Ia, among the purest of natural gems," it said.

"It's very rare to find green diamonds, even over one carat," said Max Fawcett, global head of Christie's Jewellery.

"To find something in 5 carat of this quality and this color is truly remarkable."

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 13.6 million Swiss francs ($17.3 million), 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.

Wednesday's 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 in 2003, as part of a Splendor of Diamonds exhibit.

The 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.