The failure to secure a sale for the gemstone on Thursday put a damper on what was supposed to be the climax of a glittering week of jewellery auctions.

The walnut-sized stone, shaped like a spinning top, will remain nameless as whoever buys the 103.83 carat diamond is supposed to name it.

Unable to find anyone willing to pay more than 10 million Swiss francs (6.5 million euros, 7.6 million dollars) for the jewel, Guy Jennings, president of Sotheby's Switzerland, declared the lot not sold.

Geneva, Europe's capital for gemstone auctions, hosted a series of jewellery and watch sales this week by leading auctioneers Sotheby's and Christie's, which also included the world's largest sapphire, a rare oval diamond and buckets of rubies, emeralds and pearls.

"It remains a sensational diamond. There is nothing wrong with the stone, it just clearly wasn't its moment," David Bennet, a senior consultant at Sotheby's, told reporters after the auction.

Despite the wealth of treasures on offer, the nameless diamond created the most interest at the Beau-Rivage Hotel where the auction was held, sending a buzz of excitement around the packed auction room when it was presented.

Unfortunately, this did not translate into equally enthusiastic bids and the 10 million Swiss francs offered by someone over the telephone fell short of an undisclosed minimum sale price.

"It really is not a reflection on the market," insisted Bennet, explaining that Sotheby's would likely take charge of the jewel for the time being.