The buyer is an American collector, says the auction house Sotheby's.
The diamond called Mediterranean Blue is described as "exceptionally rare". It originates from the Cullinan diamond mine in South Africa, not least known for the Cullinan diamond, the world's largest rough diamond whose largest pieces are found among the British crown jewels.
The bidding in Geneva is reported to have been intense. But some had expected an even higher price.
The broad uncertainty, including trade tensions between the US and China, may have dampened the bidders' confidence, says Tobias Kormind, head of the diamond company 77 Diamonds.
