Largest coloured diamond ever auctioned may see $1.2m at Christie's

The largest coloured diamond ever auctioned forms part of Christie's Magnificent Jewels sale later today

The largest coloured diamond ever graded at the Gemological Institute of America is to cross the block at Christie's today (December 10), as part of its Magnificent Jewels auction in New York.

The red colour's origin is not yet fully known, but is believed to be due to nitrogen or hydrogen

The circular-cut fancy reddish orange diamond is expected to sell for $700,000-1.2m as the world's largest coloured diamond ever auctioned. Weighing 3.15 carats, the stone is accompanied by a letter from the Gemological Institute of America that confirms it as the largest it has ever seen.

The diamond is prized for both its deep colouration and its large size, with stones in red and orange the rarest of all fancy coloured diamonds. The origin of the colouration is not yet fully understood, though it is believed to take place as the crystal lattice forms under the Earth's crust, spurred by the presence of nitrogen or hydrogen.

One of the world's largest red diamonds is currently housed at the Smithsonian Institution, weighing 5.03 carats. In November, the Archduke Joseph diamond, which is of flawless clarity and weighs 76 carats, sold for $21.5m at Christie's - a world record price per carat for a colourless diamond.

Diamond prices continue to rise at 2012 auctions, with a world record set in November

Expecting the highest bids of the auction is a 50.01 carat diamond ring by Graff, which is set to reach $7m-10m. Mounted in platinum, the rectangular cut diamond is accompanied by a working diagram that indicates that its clarity may be internally flawless.

Be sure to visit our Watches & Jewellery news section for the results of this sale and more of the latest from across the market.