A 10.03-carat blue diamond has sold at an auction in Geneva, Switzerland for $21.5 million, Swissinfo reports.

The “Mediterranean Blue,” which was estimated to be worth $20 million, “attracted enormous interest” at the sale, Sotheby’s said.

But Tobias Kormind, director of 77 Diamonds, Europe’s largest online jeweler, said the sale was “less impressive” than expected.

“The diamond exceeded its $20 million estimate, which suggests significant interest. But general uncertainty, including trade tensions between the US and China, may have dampened bidder confidence,” he said.

A battle between two bidders started at CHF9 million and lasted “almost three minutes.”

A private American collector won the precious stone, the auction house said, without identifying the bidder.

The diamond was recently discovered in the legendary Cullinan mine in South Africa. It was then unveiled at the beginning of April in Abu Dhabi, the capital of the United Arab Emirates.

According to Sotheby’s, “finding a blue diamond is an incredibly rare event."