Saturation

Saturation is the strength or purity of a colour, running from greyish or brownish at the weak end to vivid at the strong. Colour has three dimensions, hue, tone and saturation, and this is the one that moves price hardest. Two blue sapphires can share a hue and a tone and sit far apart on price because one carries a grey mask and the other does not. Brown does the same to ruby and to spinel. The stone that shrugs off both masks in plain daylight is the stone showing you saturation, and daylight is the operative word: dealer spot lamps and warm halogen flatter almost anything.

Saturation is also the dimension most easily confused with tone. Dark is not vivid. Plenty of dark sapphire is dark because it is heavy in tone and dull in saturation, and the market discounts it accordingly.

On a lab report

GIA's colour grading system scores saturation from 1, greyish or brownish, to 6, vivid; that scale lives in its coloured stone grading and teaching, not on a standard identification report. GRS puts trade colour names on its reports, "vivid blue" and "vivid red" among them, judged against its own reference charts. Gübelin and SSEF describe the colour in words and, for stones that qualify, add a comment that the colour may be called royal blue or pigeon's blood in the trade. The point is that "vivid" on one document and "vivid" on another are two houses' opinions, not a shared standard. Read the word with the letterhead.

Before buying, put the stone by a window on a grey card, next to another of the same hue if the dealer has one, and look for the mask. Ask which laboratory used the colour word on the report and whether the report is current. Saturation is where sapphire, ruby, spinel and tsavorite earn their price, and it is where a photograph lies most fluently.