The 8.01-carat “Sky Blue” diamond in a Cartier setting was snapped up for $17.1 million (15.9 million euros), a hike of 33 percent on the $12.8 million it sold for in 2012, said the auction house's international jewellery division director David Bennett.
   
Rival auction house Christie's opened the season on Tuesday with 167 lots that sold for $97 million, beating its pre-auction estimate of $80 million.
   
The “Sky Blue” sale came amid feverish demand for coloured stones.
   
Sotheby's had valued the ring at $15-25 million, well short of the astonishing $57.54 million Christie's fetched in May for the 14.62-carat “Oppenheimer Blue”, which remains the world record.
   
Tobias Kormind, head of 77 Diamonds, Europe's biggest online diamond jewellery retailer, said interest in coloured stones remained strong.
   
“Given the rarity of blue diamonds of this size and quality which have ever been unearthed, you'd expect every auction of this kind to be a dogfight,” Kormind said before the auction at the five-star Hotel Beau Rivage on Lake Geneva.

Rich with history

Sotheby's, however, failed to sell two other important lots.
   
The first was a parure, valued at $3 to $5 million, featuring diamonds once owned by Russian empress Catherine I. The diamonds were given to her by her husband, czar Peter the Great, who led Russia until his death in 1725.
   
In 1711, Catherine was worried that a raging conflict with the Ottoman Empire posed huge risks to Russia and told her husband to draft a peace treaty, Sotheby's said, citing historical records.
   
Catherine sent the peace proposal and all the jewels she was travelling with to the Ottoman sultan Ahmed III.
   
The sultan “accepted these and was obviously delighted, and the truce was given and the (Russian) empire was saved”, Bennett said earlier.
   
Also unsold was a diamond necklace with a detachable clasp owned by empress Catherine II — Catherine the Great — who ruled Russia from 1762 to 1796, also valued at up to $5 million.