Colour: How we see diamonds and gemstones

Colour surrounds us in the natural world, and while we often take it for granted, it has more influence on us than we may imagine. Some behavioural scientists contended that colour has an impact on our moods. Marketers frequently leverage colour to sway our buying behaviour. Stylists use colour to assert attitude. Ancient civilisations allocated colours to religious rites and cultural practices. When it comes to diamonds and gemstones, colour – or an absence of it – is a fundamental factor in a stone’s beauty, value, and uniqueness.

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Modern science maintains that all colours in the universe are founded in three elements: hue (colour), saturation (chroma) and tone (value). Hue refers to the spectral colours such as red, green, blue, and so on, that are visibly distinct from each other. As an exception, some gemstones exhibit a subtle combination of two or three colours. The secondary or modifying hue in these stones is usually nominated first, such as “purplish-pink” denoting that a slight amount of purple is mixed with a predominately pink colour.

Saturation represents the intensity, or quantity, of colour in a stone. The best way to understand this is if you take a can of blue paint and gradually stir in some white, rather than getting a new colour, the result is a lighter blue. An exception to this rule is that by adding white to red, we make pink. Red is a highly saturated pink; they are the same hue but the quantity of red colour is less in pink.

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Tone refers to how light or dark a colour is. On a scale of 0 to 10, 0 is colourless (or white), 5 is a medium grey and 10 is black. So, as the tone increases, it intensifies the “darkness” of the stone. Tone begins to impact the saturation of a stone once it reaches a percentage high enough to overpower it. This percentage varies with each colour, just as the saturation range varies between hues. For example, saturation in a yellow stone may reach as high as ninety percent, whilst in a blue stone it may only reach three percent, with the result that a small percentage increase in tone on a blue stone would have a far greater impact on the blue, resulting in the grey becoming more noticeable. Stones with a higher saturation, such as yellow, would require correspondingly higher levels of tone for the brown to be observed. When the percentage of tone exceeds the saturation, the brown or grey will actually become the body (primary) colour and the hue the modifier, for example bluish-grey.

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What we see in colourless (popularly referred to as white) crystals is simply due to the unimpeded transmission of all hues in the visible region of the solar spectrum. Colour on the other hand in a crystal is related to the presence of impurity atoms and structural defects in the gems atomic structure, both of which act as colour centers. The colour centers alter the transmission of visible light through the stone by absorbing certain spectral wavelengths or hues. The remaining lines or wavelengths of white light that the stone did not absorb are transmitted to the observer. The way in which these remaining spectral wavelengths are combined and perceived by the human eye and translated by the brain determines the colour we see.

With this knowledge, next time you admire a diamond or coloured stone, consider how the colour, saturation, and tone have formed its unique look and influenced the way you see it.

HAMISH SHARMA / Head of Important Jewels

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February 2021