Unlike diamonds there is no formal grading system for coloured stones. Because gemstone cutting is based solely on maximising the colour of the stone there are no strict rules in how it should be cut. Whereas diamonds have particular measurements and percentages that maximise the light performance and brilliance it makes it much easier to grade these as the range is much smaller.

Measuring colour in gemstones means considering the hue, tone and saturation which make up the overall body colour of the stone. The body colour refers to the gemstones basic colour determined by the selective absorption of light, this means that as white light enters the gemstone it splits into its spectral colours, most of the colours get absorbed in the stone and the rest are reflected back to the eye which is the colour that you see. To explain a bit further, hue is your first impression of the stone, it is the green of an emerald or the red of a ruby, most colours also have secondary colours which make up the overall colour so the ruby might be red with slight blue colour zoning, which makes the stone look slightly purplish overall. Stones with one pure colour are generally more valuable. Tone refers to the lightness and darkness of the stone, very dark stones might be cut quite shallow to give the stone a light ‘window’ to make the colour appear lighter. Saturation is a colours strength or intensity, for example if an amethyst is low in saturation it might appear to have a lilac colour, whereas if it has high saturation it will have the rich purple that we generally associate with amethyst. Generally the more highly saturated, the more vivid the colour and the more valuable the stone.

This chart is a 3D way of imaging the hue, tone and saturation and how they work together.

To learn more about coloured gemstones and their different qualities feel free to get in touch to speak with one of our highly trained gemstone specialists about how to choose the best stone for you.