The largest diamond in the world will go under the hammer as part of a luxury week auction by Christie's that features two stones weighing more than 200 carats each.
"The Rock", a 228.31 carat pear-shaped stone roughly the size of a golf ball, is expected to fetch between $20m (£16.2m) to $30m, the auctioneer said.
The gemstone was mined and polished in South Africa over 20 years ago and is the largest white diamond ever to appear for sale at auction. Its former unnamed owner wore "The Rock" as a Cartier necklace.
It has been shown in Dubai, Taipei and New York ahead of the live sale in Geneva on Wednesday.
According to Christie's, the previous auction record for a white diamond was a 163.41 carat gem, which sold for $33.7m in 2017.
Christie's is also selling a 205.07 carat yellow, cushion-shaped stone, estimated to sell for between $7m and $10m.
"The Red Cross Diamond" has Maltese cross faceted at its base and an unspecified portion of the auction proceeds will go to the Geneva-based International Committee of the Red Cross (ICRC).
The gem was first sold by Christie's in 1918 at a London auction where residents sold precious household items to help the war effort. Those proceeds, £10,000 (now $12,351), helped the British Red Cross Society.
A portion of the proceeds from the sale will go towards bringing clean water to people affected by conflict, an ICRC spokesperson said.
The live sale of the gemstones will start at 3PM BST and conclude at 7PM.
The auction comes at a time when the price of diamonds have been buoyed by sanctions on major producer Russia as well as the return of VIP events as coronavirus restrictions unwind across the world.
Rahul Kadakia, Christie’s international head of jewellery, said: "The Rock will join the very best of legendary gemstones which have passed through Christie’s global sale rooms since 1766.
"The market for diamonds is particularly vibrant and we are confident that this sensational gemstone will capture the attention of collectors across the globe this Spring season."
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