Grading cut coloured stones
Goal: Grade colour, clarity, cut and size consistently, and write a grading report.
Colour = hue + tone + saturation
Hue
Basic colour + modifiers: e.g. 'slightly purplish red'. GIA uses 31 hues.
Tone (0–10)
Lightness: 0 colourless → 10 black. Gem range 2–8; most top stones 5–7.
Saturation (1–6)
Strength: 1 greyish/brownish → 6 vivid.
Grade colour face-up, at arm's length, under daylight-equivalent light (5500–6500 K) on white; check under incandescent too. Brown/grey masking lowers value most.
Clarity (eye-visible standard, by type)
Judge against the gem's clarity type: an SI1 emerald is fine; an SI1 aquamarine is poor.
Cut: what you're really judging
Brilliance
% of face-up area returning light. Aim 70%+.
Windowing
Washed-out centre you can see through: pavilion too shallow.
Extinction
Dark areas that stay dark: often deep or steep pavilion. Some is normal in high-RI gems.
Proportions
Depth % 60–80% typical for coloured stones. Check for bulging pavilion (dead weight, paid by the carat).
Outline & symmetry
Even shape, centred culet, aligned facets.
Finish
Polish lines, scratches, abrasions, extra facets, wavy girdle.
Native cuts
Re-cutting can add value but loses weight; calculate before deciding.
Phenomenal stones
Members can use the Grading card tool: it walks you through every factor and outputs a standard report line.
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