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.

It’s pre-sale value? $15 – 25 million.

Chairman of the 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, a 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,” he added.

Among other items to be auctioned by Sotheby’s are gems once owned by Russia’s Catherine the Great, who lived in the 18th century.

The jewels will all be auctioned in Geneva in November.