The world's largest fancy vivid orange diamond is the star lot in Christie's November 12 auction of Magnificent Jewels in Geneva.
The diamond, known simply as The Orange, weighs in at 14.82 carats and is expected to see up to $20m. Orange diamonds, sometimes called fire diamonds, are incredibly rare at auction, with the largest to have appeared so far weighing just six carats.
The last time a fancy vivid orange diamond sold at auction was at Sotheby's in 1997, with the 5.54 carat "Pumpkin Diamond" selling to Ronald Winston (son of famed jeweller Harry Winston) for $1.3m.
The Gemological Institute of America said of The Orange diamond: "In the Laboratory's experience, strongly coloured diamonds in the orange hue range rarely exceed three or four carats in size when polished. (This diamond) is almost four times larger than that size range.
"In GIA's coloured diamond grading system, as the colour appearance of strongly coloured diamonds transitions from orangy yellow to orange the occurrence becomes progressively more rare, that is, the less yellow present the more rarely they occur."
Also featuring in the auction will be a 58.29 carat sapphire that has been lavishly mounted in a "Cote d'Azur" multi-gem brooch by Anna Hu, jeweller to the stars. It is valued at $2.5m-3.5m.
The auction comes as one of a number of high-profile diamond sales that have swept the market recently, with Sotheby's setting the world record for a white diamond at $30m in Hong Kong.
On November 13, Sotheby's will offer the Pink Star diamond, which is expected to set a new record for any diamond ever sold, at $60m.
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