A very rare blue diamond discovered in South Africa last year could fetch up to $55 million at an auction next month.

The Blue Moon diamond, among the largest known fancy vivid blue diamonds, will be the centrepiece of Sotheby’s November 11 auction of Magnificent Jewels and Noble Jewels in Geneva, Switzerland. 

This diamond weighs 12.03 carats and has been graded Fancy Vivid Blue – the highest possible colour grading for blue diamonds – by the Gemological Institute of America (GIA). The cushion brilliant-cut stone also boasts exceptional clarity, having been declared Internally Flawless by the GIA. 

This rare diamond comes to the market with an estimate of $35-55 million.

David Bennett, Worldwide Chairman of Sotheby’s International Jewellery Division, says: “The tremendous excitement last year over the discovery of a 29.62-carat blue rough diamond at the Cullinan mine in South Africa has now been proved to have been totally justified. Weighing in at 12.03 carats, the Blue Moon diamond is a simply sensational stone of perfect colour and purity, combined with a superb cushion shape. Blue, for me, is the most mysterious and magical of all the colours of diamond, and the Blue Moon will now take its place among the most famous gems in the world.”

Petra Diamonds, which owns the Cullinan mine, announced the discovery of the rough blue diamond in January last year. The mine is one of very few sources in the world for blue diamonds, but these rare gems still only account for a minute fraction of the diamonds unearthed at the mine. 

The rough was purchased by New York’s Cora International which cut and polished the Blue Moon, a highly skilled and complex process that took over six months.

The Blue Moon Diamond’s name refers to the expression ‘once in a blue moon’, which is used to describe something which happens only extremely rarely.

A blue diamond is created when boron is mixed with carbon during the gem’s formation. Scientists from the National Gem and Mineral Collection at the Smithsonian Institution were fascinated by the extreme rarity of the Blue Moon. 

They were able to study the diamond ahead of its being exhibited in the Gem Vault of the Natural History Museum of Los Angeles County. It has been reported that, under testing, they observed that the colour of the Blue Moon is “true and saturated” throughout the diamond, with no other colours present. Dr Eloïse Gaillou, Mineral and Gem Curator at the Natural History Museum of Los Angeles County, noted the “intense ocean blue color” and placed the Blue Moon “on the top of the rare list due to its colour, clarity and size”.

According to the GIA Monograph, the hue of the Blue Moon is “likely to have never before been seen within such a large diamond, or any gemstone”, and “could be indescribable to even the most experienced diamantaire or colour theorist; some, however, liken it to the ocean”.

Sotheby’s Geneva has a distinguished and successful history of selling exceptional diamonds under the hammer of David Bennett. In November 2010, The Graff  Pink - a Fancy Intense Pink Diamond weighing 24.78 carats, sold for $46,158,674  ($1,862,739 per carat) – establishing a new world auction record for a diamond and any jewel.

Sotheby’s also holds the world auction record for a blue diamond, which was established by the “Zoe Diamond”, a Fancy Vivid Blue Diamond weighing 9.75 carats, sold for $32,645,000 at Sotheby’s New York in November 2014. The stone also set a new world auction record per carat for any diamond, at $3,348,205 per carat.

Among other exceptional diamonds sold at Sotheby’s, a magnificent 118.28-carat oval diamond was sold at Sotheby’s Hong Kong in 2013 for $30,600,000 ($258,708 per carat), setting a world record for a colourless diamond.

Sotheby’s has been uniting collectors with world-class works of art since 1744. Sotheby’s became the first international auction house when it expanded from London to New York (1955), the first to conduct sales in Hong Kong (1973), Geneva (1980), India (1992) and France (2001), and the first international fine art auction house in China (2012). 

Today, Sotheby’s presents auctions in 9 different salesrooms, including New York, London, Hong Kong, Paris, and Geneva and Sotheby’s BidNow program allows visitors to view all auctions live online and place bids in real-time from anywhere in the world. Sotheby’s offers collectors the resources of Sotheby’s Financial Services, the world’s only full-service art financing  company, as well as private sale opportunities in more than 70 categories, including S|2, the gallery arm of Sotheby's Contemporary Art department, as well as Sotheby’s Diamonds and Sotheby’s Wine. Sotheby’s has a global network of 90 offices in 40 countries and is the oldest company listed on the New York Stock Exchange (BID).