Tone

Tone is how light or dark a colour is, independent of how pure it is. Fine coloured stones tend to sit in the medium range, dark enough for the colour to register and light enough for it to breathe. Push too dark and extinction eats the display, the facets go black at the edges and the stone reads as ink. Go too light and the colour thins to a wash. Blue sapphire from Ceylon leans light, Australian and much Thai material leans dark, and Kashmir and the best of Burma sit in the middle, which is one reason they cost what they cost. Ruby follows the same logic; so does spinel, where the finest reds are lighter in tone than most buyers expect.

Tone and saturation are different measures. A dark stone is not necessarily a strong-coloured one. Separate the two and half the trade's colour vocabulary becomes clearer.

On a lab report

GIA's colour system scores tone from 0 to 10, colourless to black, and its coloured stone graders work between very light and very dark; the number is a grading and teaching tool rather than a line on a standard report. Trade names bundle the two measures together. Royal blue, on a GRS report or in the "may also be called royal blue in the trade" comment that Gübelin and SSEF add, describes a stone in a defined band of tone and saturation. Useful shorthand. But the report's own colour description is the better record, because it will still mean the same thing when the fashion for a name has moved on.

Look at the stone off the white tray, on a grey background and against your hand, since white paper makes everything look lighter than it wears. Ask the dealer to place it in tone terms, light, medium or dark, and see whether the answer matches what the report says.