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Fancy Coloured Diamonds - The Facts of Colour Grading

Posted on: 06/06/2016

The world’s largest fancy intense blue diamond, the ‘Cullinan Dream’, is being auctioned as the centrepiece of Christie’s Magnificent Jewels and The Cullinan Dream action in New York this Thursday. The impressive 21.18ct blue diamond was cut from a 122.52ct rough diamond found in 2014 and is expected to fetch up to £19.8m!

Those of you who keep up with your diamond auction news may have noticed, however, that this is substantially less than the whopping £39.5m total fetched by the 14.62ct Oppenheimer Blue diamond just a few weeks ago. The Oppenheimer Blue is only around two thirds of the carat weight of the Cullinan Dream so why did it fetch twice as much?

The answer lies in a closer look at the colour grading of fancy diamonds. While, the cut, clarity, and carat weight of fancy diamonds are assessed in very similar ways to colourless diamonds, the colour is rated very differently. (Although the clarity of fancy diamonds can also be affected by variations in a stone’s colour as colour grading is considered to be an inclusion, clarity is still much less important for fancy diamonds than it is for colourless ones as the colour is the main feature.)

The colour of fancy diamonds is assessed very differently to other diamonds. This is because normally diamonds are graded according to how colourless they are. Top rated diamonds are given a ‘D’ grade, which is also known as ‘colourless’. From there, the further down the alphabet you go, the more colour creeps into the stone as it goes lower down the scale until the last few letters of the alphabet are applied to stones with an S-Z grading, which means light yellow. Obviously this scale isn’t very helpful, however, when you’re trying to assess the intensity of colour in a diamond instead of its absence!  Coloured diamonds are therefore graded according to a slightly different scale. The example from the Gemological Institute of America below demonstrates the colour grading for blue diamonds and shows why they are so much more complicated. On this scale, the least valuable colour ratings are shown at the top left of the diagram, labelled ‘faint’, while the strongest and most valuable fancy diamonds are towards the right, called ‘fancy vivid’ or sometimes ‘fancy deep’.


Fancy Blue Diamonds - tone and saturation chart from the GIA

Instead of a straightforward scale from colourless to coloured, colour scales for fancy diamonds have to consider three factors:

  1. Hue - This refers to the type of colour. Fancy diamonds come in a wide range of colours, ranging from the most common (fancy yellow) to the incredibly rare (such as fancy red, of which there are only four recognised examples in the world). Blue is one of the rarer hues.
  2. Saturation - This means assessing the intensity of the colour by looking at how strong or weak it is. The stronger the colour, the further to the right-hand side of the scale above it will be located and the more rare and valuable it will be.
  3. Tone - This refers to the relative light or darkness of a stone and affects whether it is termed, for example, ‘Fancy Intense’ or ‘Fancy Deep’, placing it towards the top or the bottom of the scale above.

If we compare the two big blue diamonds side by side we can directly see the effect this colour grading has on the value of the gemstones.

Name: Cullinan Dream

Carat Weight: 21.18ct

Colour Grading: Fancy Intense Blue

(Estimated) Price at auction: £19.8m

Name: Oppenheimer Blue

Carat Weight: 14.62ct

Colour Grading: Fancy Vivid Blue

Price at auction: £39.5m

As you can see, the Oppenheimer Blue, despite being smaller than the Cullinan Dream, has a higher colour rating of ‘Fancy Vivid’, making it a much rarer colour and leading to a substantial increase in value. The Cullinan Dream is estimated to only fetch half as much because it is a step further down on the colour scale at ‘Fancy Intense’.

Of course, the Cullinan Dream is still estimated to fetch nearly £20m, which is quite a bit more than casual pocket change to most people, but just think what it might have been worth if it had been graded ‘Fancy Vivid’ instead!

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