Blue diamond necklace of Peter the Great’s wife called ‘Sky Blue’ the is up for auction at Sotheby's. The precious stone weighs more than 8 carats. It is mounted on a ring by Cartier jewelers. Diamond "Blue Sky" is expected to raise about $12-52 million.

Two ornaments of Russian empress will be the highlight of the auction. One of them is the necklace of Catherine the First. Presumably, the empress

received it as a gift from the Ottoman Sultan Ahmed the 3rd in 1711, when the latest sighed the Prussian peace treaty.