Hue, tone and saturation
Gemmologists describe a coloured stone's colour with three numbers and letters: which colour it is (hue), how light or dark (tone), and how strong or greyish (saturation). Build a colour and read it the way a grading report writes it, for example slpR 6/5.
The stone is ray traced, so its facets show light and dark areas around the body colour; the flat chip shows the colour on its own. Neither is a colour reference: screens differ.
30 hues around the wheel, from red through orange, yellow, green, blue, violet and purple, and back to red. Tap a segment.
Pink and brown are described separately from the wheel.
The three parts
Hue
The basic colour: one of the named hues around the wheel. A main hue can carry a modifier, from "very slightly" to "strongly", towards its neighbour: slightly purplish red, strongly yellowish green.Tone
How light or dark the colour is, from 0 (colourless) to 10 (black). Gems are described between 2 (very light) and 8 (very dark).Saturation
The strength of the colour, from 1 (greyish or brownish) to 6 (vivid). Low saturation looks dull or "masked"; high saturation looks pure and intense.Writing it down
The hue code comes first, then tone and saturation after a slash: slpR 6/5 is slightly purplish red, tone 6, saturation 5. Grading is done against physical colour references under controlled light, never from a screen.The scales
Sources
The GIA colour description system as set out in the Gemology Project textbook, Colour grading (LibreTexts, CC BY-NC-SA): the 31 hue names and codes, the 0–10 tone scale with 2–8 used in practice, the 1–6 saturation scale, and the notation (for example B 5/2). GIA, Colored diamond grading and the GIA coloured stone grading course: colour described by hue, tone and saturation; hue the characteristic colour, tone its lightness or darkness, saturation its depth or strength.
Colours on this page are computed for display (hue angle, lightness from tone, chroma from saturation) and are illustrations. GIA's comparators are physical references; no screen colour corresponds to a grade.