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”.
