“The Mediterranean Blue”, a fancy vivid blue diamond weighing 10.3 carats with an estimated value of $20 million, attracted an intense bidding battle.

Bidding began at nine million Swiss francs ($10.8 million), with a fierce back and forth before the diamond was ultimately sold to a private US collector, whose name was not given, for 17.9 million francs ($21.5 million), Sotheby’s said.

The Mediterranean Blue, which is a brand-new blue diamond recently mined from the legendary Cullinan mines of South Africa, generated huge excitement within the diamond industry ever since it was first announced in March, the auction house said.

Ahead of its final showing in Geneva on Tuesday, it was unveiled as part of a Sotheby’s debut exhibition in Abu Dhabi last month, where it was showcased alongside seven other “extraordinary” diamonds and gemstones collectively worth over $100 million.

“At the top of the rarity pyramid are blue diamonds,” Quig Bruning, head of jewels for Sotheby’s in North America, Europe and the Middle East, said at the Abu Dhabi show.

After serving as auctioneer at Tuesday’s event, he hailed the gem as “undoubtedly the defining stone of the season”, saying in a statement that it “ranks among the top blue diamonds we have sold”.

Tobias Kormind, head of Europe’s largest online diamond jeweller 77 Diamonds, was less upbeat, describing the sale as “less dazzling than anticipated”.

“The diamond did exceed its $20 million estimate, suggesting there was meaningful interest,” he acknowledged.

“But broader uncertainty, including the ongoing US-China trade tensions, may have dampened bidder confidence and muted what could have been a more frenzied atmosphere.”