A rare fancy vivid blue diamond went on display at Sotheby's in London on Tuesday. Known as 'The Sky Blue Diamond' it weighs 8.01 carats and is mounted on a diamond ring by Cartier.
Its pre-sale value? $15 - 25 million (£12 - £20 million).
Chairman of Jewellery Division at Sotheby's, David Bennett, says: "Blue diamonds are very much in the press these days.
"This time last year we sold the famous Blue Moon of Josephine, 12 carat cushioned vivid blue diamond, for $4million a carat. Extraordinary sum.
"This year we have this very beautiful ring. It's set with an eight carat emerald cut diamond. Emerald cut I think is the most flattering way of cutting coloured diamonds."
Among other items to be auctioned by Sotheby's are gems once owned by Russian royalty. Catherine the Great, who lived in the 18th century, is said to have commissioned this necklace.
Bennett, saying: "In the 1920's the Soviets decided to dispose of some parts of the Russian crown jewels and this necklace and brooch was sold in London in 1927 and were purchased and then resold to the family of the previous owner and so it's extraordinary that they're now appearing again at auction. They're part absolutely of Russian history."
The jewels will all be auctioned in Geneva in November.