"It really is an exceptional gemstone, a true 'vivid', in other words one with a particularly intense colour that captures the light on all its facets," Jean-Marc Lunel, head of the jewellery division at Christie's auctioneers in Geneva told AFP during a viewing.
Described by the auctioneer as a "the largest fancy vivid orange diamond in the world", the stone is 14.82 carats and is the size of an almond.
Pure orange diamonds, also known as "fire diamonds", are exceptionally rare in nature and very few have been auctioned, with the largest never more than six carats.
The stone will be on sale next Tuesday as part of Christie's traditional November jewellery auction in Geneva.
The following day, rival auctioneers Sotheby's are to sell a flawless 59.60-carat pink diamond, which has an estimated price of USD 60 million.