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According to the GIA there are 31 hues.

When valuing a coloured gemstone, most people in the industry will agree that it is the colour of the gem that is its most important evaluation criteria. Colour can be broken down into three components: hue, tone (lightness) and saturation (colourfulness).

‘Hue’ is used to describe the element of colour through red, green and blue. One way of understanding hues is to imagine that in painting, without the addition of black or white, an artist can create most hues by mixing the three primary colours of red, yellow and blue. Reddish orange and greenish blue are both hues. There are just 31 hues that the GIA use to describe nearly all coloured gemstones. Printed pages in books such as this are not always entirely accurate in terms of colour, so rather than show these 31 shades, I have opted to give you a list of these hues on the following page and suggest that as you read them, you allow your mind to visualize each hue. Next to each hue you will see the letters that are used by the GIA as a short code: this may be useful if you ever have your gemstones valued, as you will often see the codes on your valuation certificate. There are two generally accepted additions to the chart; Pink (Pk) which is a light red and Brown (Brn).

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According to the GIA there are 31 hues.