The jewel — the largest top-quality, brilliant-cut white diamond ever to appear at auction — had been valued by Sotheby's at between US$13.3 million and US$17.8 million (€9.05 million and €12.1 million).
The winning bid on Wednesday made the diamond the second most-expensive stone or piece of jewelry ever sold at auction, falling just short of the US$16.5 million fetched by a 100.1-carat diamond at the same Sotheby's branch in Geneva in May 1995.
Marciano's purchase, made by telephone, came to US$16,189,769 (€11,013,448) including the buyer's premium.
"In 32 years, it was this stone that had the ability to take my breath away," said David Bennett, the auction house's chairman of jewelry for Europe and the Middle East. "It had so much life and brilliance."
Bennett identified the seller as Ron Cohen, the owner of the Los Angeles-based Clean Diamonds.
Marciano, who named his diamond "Chloe" after his daughter, lives in Beverly Hills, California.
Some 300 collectors, gem manufacturers and those just curious packed a salon in Geneva's historic Beau Rivage hotel for the sale, which pulled in just under US$57 million (€38.8 million) in total.
A fancy, vivid blue diamond ring sold for over US$4.7 million (€3.2 million), beating the high-end estimate, as did a marquise-shaped diamond ring by Cartier that fetched more than US$2.3 million (€1.6 million).
Sotheby's said it sold 307 lots, against 78 unsold.
"We had interest from all over the world," Bennett said. "This sale said a lot about the jewelry market internationally, especially the diamond market. It says great things about jewels."
Avraham Eshed, an Israeli gem manufacturer in attendance, was less reserved.
"Diamond is booming and prices are going to continue to rise," said Eshed, the president of Eshed Diam Ltd. "There's more demand than good in the world market, especially in higher carats."
He said the market was being driven increasingly by clients in emerging economies such as India, Thailand, Hong Kong and Taiwan. But stones are rare: even some bids 5 percent to 7 percent above market value are being turned down for precious gems coming out of Africa, he said.
Bennett said the Kimberly Process aimed at combatting blood diamonds was respected at every stage for the white diamond.
Cohen said he purchased it from Angola's national diamond company two years ago, and knew immediately he had found something special.
"The size and especially the quality of the rough material was extraordinary," he told The Associated Press by telephone. "I only hope I will one day come across another stone like that one."
Marciano's diamond — showcased in Hong Kong, Paris, New York, Rome, Los Angeles, London, Dubai and Bahrain before the auction — received the highest possible grading, Sotheby's said. It is D-color, or finest white, has flawless clarity and its cut, polish and symmetry have all been graded excellent, it said.
The overall sale makes 2007 Sotheby's strongest year for jewelry since 1995, Bennett said.
It comes only a week after disheartening results from the New York-based company's Impressionist and Modern Art auction, which sent Sotheby's stock tumbling.
That sale last week fetched US$269.7 million (€185 million), well short of estimates, and led some analysts to speculate whether the flight from risk currently gripping financial markets may be spilling over into the art world.
