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.
10 incredible auctions
10 incredible auctions
The most expensive watch ever sold at auction fetched just under $24 million in November 2014. The gold pocket watch was made by Patek Philippe, and is the most complex ever made without the use of computer technology.
The Henry Graves Supercomplication was commissioned in 1925, and took eight years to make.
The world's most expensive stamp sold at auction in 2014 for over $9 million.
The British Guiana One-Cent Magenta is as rare as a stamp can get. British Guiana was one of the first countries in the New World to start issuing stamps, but in 1856, they ran out, and asked the local newspaper printer to produce extras.
There were two denominations: the four-cent, which is very rare, and the one-cent - of which only one has ever been discovered.
In May 2015, an anonymous London businesswoman snapped up the licence plate KR15 HNA for £233,000, making it the most expensive standard number plate ever to be sold in the UK.
Queen Victoria's bloomers sold at auction for £6,200, along with a pair of her silk stockings.
They have a 52-inch waist, and belonged to the monarch in the 1890s - "towards the end of her life when she had eaten a lot more than most people could afford to," said auctioneer Michael Hogben. In today's sizing, they'd be a size 26.
In 2014, a three-year-old slice of cake sold at auction for $7,500 (£4,800). The reason the stale cake was in such demand was that it was from the marriage of Prince William to Kate Middleton in 2011.
The buyer said he intended to give it away as part of promoting his Silicon Valley start-up.
A British coin sold at auction for a record-breaking £430,000 in 2014. After fees, the buyer paid £516,000 - making it the most expensive modern British coin ever to be sold.
The coin is only one of two in existence. It was a 'proof' for a gold sovereign which was meant to be produced to commemorate the coronation of Edward VIII in 1937. However, Edward abdicated in 1936, so the coronation never happened and the coins were never made