The world's largest cut diamond, the Pink Star, is expected to sell for up to £40 million tonight but what makes it so special?
The world's largest cut diamond, the Pink Star will go up for auction on Wednesday night in Geneva where it is expected to break records.
It is expected to be the world's most expensive diamond when it goes under the hammer as the 59.6-carat 'fancy' diamond is valued at £40 million.
'Fancy' coloured diamonds are some of the rarest gems in the world- so-called because of the intensity of their colour.
The Gemological Institute of America (GIA) has devised a colour grading system which is used as the standard for grading the colour of diamonds.
The most valuable diamonds tend to have the least amount of colour. Diamonds that are 100 per cent colourless get the highest rating- a letter D grade.
The system continues through the alphabet to the letter Z, and diamonds decrease in value as the colour becomes more obvious.
Diamonds get the lowest Z rating if they are yellow or brown in colour and these tend not to be sold as gemstones.
However exactly the opposite happens with 'fancy' coloured diamonds, their value increases with the strength and purity of their colour.
Laboratories use a list of 27 colour hues that span the full spectrum for coloured gems and diamonds and the saturation of these hues are described with one of nine descriptors.
'Fancy vivid’ is the highest possible colour grade that can be given.
Most 'fancy' coloured diamonds are muted and small, so large, 'vivid fancy' coloured diamonds are extremely rare and particularly valuable.
Most coloured diamonds will also have tints of secondary colours which decreases their value.
According to the GIA, 'fancy' coloured diamonds are classified as yellow and brown diamonds that exhibit colour beyond the range of the colour grading system.
However they can also be diamonds that exhibit any other colour when they are placed face up.
When using the D-Z scale diamonds are examined face down but coloured diamonds are graded using a different procedure and are examined face up.
'Fancy' diamonds come in every colour of the spectrum, including, most importantly, blue, green, pink, and red.
Red, green, purple, and orange tend to be the rarest colours followed by pink and blue. The most common colours are yellow and brown so they are often less valuable than rarer coloured diamonds.
White, black and grey diamonds are also considered 'fancy' diamonds.
The 'Pink Star' gets its intense pink colour due to changes to the electron structure, known as 'plastic deformation' during its journey to the earth's surface.
The auction record for any jewel sold was also set by a pink diamond.
The 24.78 carat Graff Pink which was bought by Laurence Graff for £28.5m in 2010.
However the Pink Star's extraordinary size and rich colour are unparalleled.
David Bennett, chairman of Sotheby's Switzerland which is auctioning the diamond, said the Pink Star is "of immense importance".
Yesterday the largest vivid orange diamond ever sold at auction went for £22 million when it was auctioned by Christie's in Geneva.
The 14.82 carat diamond known as 'The Orange' was expected to sell for just half of that price but made a staggering £1.5 million ($2.4million) per carat.
According to Christie's this is a world record price per carat for any coloured diamond sold at auction.
The vivid orange colour is suspected to be the result of the facedown of nitrogen in the diamond's creation.
