An extremely rare 100-carat flawless diamond the size of a walnut fetched $22.1 million at Sotheby's Magnificent Jewels auction in New York City on Tuesday.

The classic emerald-cut colorless diamond, which was mined by De Beers in South Africa in 2013, was originally 200 carats in weight in its raw, unpolished form before the company spent more than an entire year polishing and shaping the gemstone to its current size.

Flawless diamonds, especially diamonds that are bigger than an entire knuckle, are very rare. They are classified as such because they are completely free from internal defects, even when subjected to intense magnification. To date, only six flawless diamonds weighing more than 100 carats have been sold in the last six years, including the latest one sold by Sotheby's and four others by the same auction house in Geneva and Hong Kong.

"People everywhere have been drawn to it from across the room and they are in awe of its size, particularly when they put it on their hand," says Gary Schuler, head of Sotheby's jewelry department in New York. "They can't believe there's a diamond this pure of such impressive scale. It's almost like looking at the glimmer of a reflecting pool."

Three private jewelry collectors bid for the massive diamond, with the initial bid starting at $12 million and going as far up as $19 million in less than four minutes into the auction. In the end, a private buyer speaking with a Sotheby's representative over the telephone placed the winning bid on Lot 357 for $19.5 million, leading to the total final price of $22.1 million, including buyer's premium.

The price falls short of the $30.6 million fetched by the 118.28-carat oval-shaped flawless white diamond Sotheby's sold in Hong Kong in 2013. It is extremely far behind the $83.02 million for the 59.60-carat Pink Star diamond, which holds the world record for the price of a gemstone. The Pink Star diamond is now part of Sotheby's inventory though, after the winning bidder defaulted on his bid.

The 100-carat diamond was the centerpiece of Sotheby's Magnificent Jewels collection of more than 350 pieces, for which the auction house raised more than $65 million. Other remarkable pieces include a rare pendant-necklace of platinum, emerald, sapphire, lapis lazuli, and diamond designed by Art Deco designer Charles Jacqueau, dubbed "the Picasso of jewelry design," for Cartier. The necklace sold for $2.6 million, past its pre-sale estimate of $1.8 million to $2.2 million.

Schuler says online buyers contributed "heavily" to the auction. The final four lots comprising the Spectacular Bid Collection of four jewelry pieces made of cobalt-blue sapphires and white diamonds were sold to online bidders for $1.3 million.

"It was encouraging to see the impact of private collectors throughout the day, including at the highest end of our sale," Schuler says. "Eight of our top 10 pieces were purchased by private individuals."

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