Earlier this week, a rare blue diamond was sold for $17.1 million during Sotheby’s autumn jewel auctions in Geneva. The 8.01-carat diamond, appropriately named “Sky Blue” and presented in a Cartier setting was valued at $15-25 million by the auction house and had been previously sold in 2012 for $12.8 million – gaining 33 percent in value in four years.

Despite this success, “Sky Blue” pales in comparison with another blue diamond sold by Christie’s, highlighting the intense rivalry between Sotheby’s and Christie’s. The 14.62-carat  “Oppenheimer Blue”, sold in May at a Christie’s auction for $57.54 million – which remains the world record price.

“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,” said Tobias Kormind, head of 77 Diamonds, Europe’s biggest online diamond jewellery retailer.

It wasn’t all sunny skies at Sotheby’s though, as the auction house did not manage to find buyers for two valuable items.

First is a parure that used to belong to Russian empress Catherine I. Valued at $3-5 million, it features diamonds and has, shall we say, a quite interesting story.

The other unsold piece also belonged to the Russian imperial family, this time Catherine II, also called Catherine the Great. The diamond necklace, fitted with a detachable clasps, was valued at $5 million and you can read more about it in our earlier story via the link above.

Christie’s, which opened the season on Tuesday with 167 lots, achieved $97 million in sales, and beat its pre-auction estimate of $80 million. Sotheby’s, on the other hand, were expecting total sales in excess of $100 million this season.