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As discussed elsewhere, colours are made up of three elements: hue, saturation and tone. ‘Tone’ is used to describe the depth of color, ranging from colourless to black, i.e. the  lightness  or  darkness  of  the  stone.  When describing the tone of a gemstone we will often use words such as ‘very light’, ‘medium-light’, ‘medium’, ‘medium-dark’, and ‘very dark’.

Tone  is  effectively  describing  the  intensity  of  the  colour.  Take  Blue  Topaz  as  an  example: Sky Blue Topaz is a light blue tone and London Blue Topaz  is  a  medium  dark  tone. Think  of Blue  Sapphire:  which  varies  from  very  pale  blue, to vivid Ceylon Blue Sapphire, through to  Midnight Blue Sapphire, where the blue is only  just detectable even with strong lighting. If you  were to have a coloured gemstone valued by the  GIA, you would find that they would grade the  tone with a numerical value: 1 being “extremely  light” through to 9 being “extremely dark”.

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A medium to medium dark Blue Sapphire.