The vast majority of diamonds in jewellery are a type of diamond known as "Cape diamonds", and these naturally fall on a colour scale of colourless to yellow or brown in varying intensities. These are described using that D-Z scale as standard and as the diamonds become less colourless, the desirability and value decreases. That is, however, not the case when it comes to fancy coloured diamonds.
When looking at fancy coloured diamonds, the rarer the colour, the more desirable the diamond. We are, however, talking explicitly about naturally coloured diamonds; colour treated diamonds are nowhere near as rare and valuable as their natural counterparts. Diamonds may be found in almost any colour under the sun, and within this there is huge variation in the appearances of each different colour. As such, fancy coloured diamonds are described using the following categories:
- FAINT
- VERY LIGHT LIGHT
- FANCY LIGHT FANCY
- FANCY INTENSE FANCY VIVID FANCY INTENSE FANCY DARK
These take into account the:
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Hue: the primary colour of the diamond.
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Saturation: the intensity of the colour.
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Tone: how light or dark the hue is.
They may be described as a combination of two colours, such as ‘purplish pink’, which indicates that the primary hue is pink, but that it has a purple tint as a secondary hue.
Generally speaking, when it comes to fancy coloured diamonds, the colour is the most important factor and determinant of value. A fancy coloured diamond may have a relatively poor clarity grade, when compared to what you may look for in a colourless diamond, yet still be a highly prized stone. You may also look out for ‘colour graining’ which is considered to be an inclusion, as it creates an uneven appearance in the colour. The cut remains important, as it helps to accentuate the colour features of the stone, but still the colour is the supreme contributor to value.
Natural fancy coloured diamonds form in a range of colours due to a number of different factors:
Red
Red diamonds are the rarest natural fancy coloured diamonds, with only 20 to 30 exceptional examples known of. Indeed, between the years 1957 and 1987 there were no GIA reports issued for red diamonds! In 2001, Mousaieff, an Israeli jeweller based in London, purchased the largest known red diamond, a red trillion cut diamond weighing 5.11carats, now known as ‘the Mousaieff red’ with an estimated worth of $20 million. They are extraordinarily rare and highly prized. They have been found mostly in Australia, Africa and Brazil, however most have been of a smaller size of one carat or less. The red colour is thought to be formed by a process called, ‘plastic deformation’. This is a geological process which occurs when the diamond crystal is put under intense stress whilst it is within the earth’s crust. The volcanic activity exerts huge pressure, which permanently changes the crystal lattice of a diamond, resulting in a change of colour. The same is thought to be the case with the formation of pink diamonds. The resulting changes in the crystal lattice structure encourage an increase in absorption of some wavelengths of light, and reflections of others - in the case of red diamonds, red light is reflected and consequently visible to us.
Pink diamonds have been most famously associated with the Argyle mine in Australia. For a long time, this was thought to be the only location where they could be found and has certainly always been considered to be the only consistent source of pink diamonds. This mine is now, however, closed, and as such the rarity value of pink diamonds has soared. Since then, they have been found in South Africa, India, Brazil, Russia and Canada, although they still remain incredibly rare. Their pink colour is also thought to be a result of 'plastic deformation', similarly to red diamonds. The most expensive price paid for any gemstone at auction was for the Pink Star, a 59.60carat internally flawless oval cut fancy vivid pink diamond, which sold at Sotheby’s Hong Kong in 2017 for $71.2 million!
Natural blue diamonds range from a light to a very deep blue, commonly with a secondary hue of grey, green or violet. They have been primarily found in the Cullinan mine in South Africa, the Argyle mine in Australia and the Golconda mine in India, and these are also exceptionally rare diamonds. Blue diamonds are a special type of diamond, a Type IIb, which means that there are impurities of Boron present throughout their chemical structure. It is this Boron that gives them their blue colour, in addition to other characteristics such as their unique ability to conduct electricity. Possibly the most famous example is the Hope Diamond; a 45.52ct fancy dark grayish-blue, believed to have been discovered in India and first recorded in 1666 by French gemmologist Jean-Baptiste Tavernier. Most recently, it was donated to the Smithsonian Museum, Washington DC, by Harry Winston.
Natural green diamonds are formed as a result of being exposed to radioactive materials such as uranium or thorium, as they form in rocks within the earth’s crust. The process of irradiation displaces electrons or carbon atoms out of the lattice, resulting in a green hue. Often, this is present as a shallow skin on the outside of the stone, but those that present a uniform colour
Yellow and brown diamonds form naturally due to the presence of nitrogen impurities within the chemical structure. Depending on how spread out these nitrogen atoms are within the diamond lattice, how many there are and whether or not they are positioned alongside fellow nitrogen atoms, the diamonds may be colourless, pale yellow, intense yellow or brown. These are Type 1a and Type 1b diamonds and are extremely common, representing the vast majority of naturally formed diamonds.