A rare blue diamond sells for more than $21 million at a Swiss auction.

 A large blue diamond weighing 10.03 carats was sold at auction in Geneva for $21.5 million, Sotheby's announced Tuesday.

The auction house said in a statement that the diamond, dubbed the "Blue Jewel of the Mediterranean," which was estimated at $20 million, "generated tremendous interest" during the auction.

However, Tobias Kormind, director of 77Diamonds, Europe's largest online jewelry retailer, said in a statement that the auction was "less impressive" than expected.

The diamond's price exceeded its $20 million estimate, indicating significant interest in the piece. However, general uncertainty, including trade tensions between the United States and China, may have dampened bidders' confidence, Kormind said.

The bidding war began at 9 million Swiss francs and lasted "about three minutes."

The auction house said the gem was purchased by a private American collector, whose identity was not disclosed.

Quig Brunning, chairman of Sotheby's jewelry division for North America, Europe, and the Middle East, said after the auction, "This gem is undoubtedly the star of the season and one of the finest blue diamonds we have ever sold."

The diamond was recently discovered at the famed Cullinan mine in South Africa. It was unveiled in early April in Abu Dhabi, and Brunning confirmed that "the blue diamond is at the top of the rarity pyramid."

According to Sotheby's, "finding a blue diamond is an extremely rare event."