This is a general methodology page. Material entries do not repeat it; they link here when it matters.
The short answer
The "4Cs" apply to diamonds and cannot be transferred to coloured stones. Apart from a few materials such as diamonds, coloured diamonds and pearls, most coloured stones have no common grading scale — only descriptive trade terms.
What "quality factors" means
Quality factors usually mean colour, clarity, cut and weight (the diamond "4Cs"); coloured stones add factors such as symmetry and proportions, and optical effects. The key point is that how these factors apply varies by material, and not every material has a written grading scale.
Materials that do have a written grading system
This table scrolls sideways on narrow screens.
| Material | Grading system | Notes |
|---|---|---|
| Colourless to light-yellow diamond | D-to-Z colour grade + clarity + cut + carat | The diamond-trade standard; colourless diamonds are rare, so higher colour grades are more sought after |
| Coloured diamond | GIA coloured-diamond colour grading (described by hue, tone and saturation) | A separate system from the D-to-Z scale for colourless diamonds |
| Pearl | Lustre, nacre thickness, surface, shape, matching and so on | See the CIBJO Pearl Book and GIA's pearl material |
| Other coloured stones | No common grading scale | See the next section |
Basis: GIA Gem Encyclopedia, "Diamond Quality Factors"; GIA's article on coloured-diamond colour grading in the GIA laboratory, Gems & Gemology, Winter 1994; the CIBJO Pearl Book.
Coloured stones have no common grading scale
This is what shoppers most often misunderstand, and what sellers most often over-promise:
- the value of a coloured stone depends heavily on the quality of the colour itself, and "what counts as a good colour" differs widely between varieties — there is no single yardstick across varieties;
- the trade relies on descriptive terms (depth of colour, lightness, presence of a secondary hue) and trade conventions rather than grade numbers;
- origin can influence market preference and price, but a locality name is not a quality grade (see Origin determination and its limits);
- treatment (heating, filling, dyeing, irradiation) can strongly affect value and must be disclosed.
On "how much does colour weigh"
A ratio such as "colour is about 60% of a coloured stone's value" is often repeated in the trade. Mind what kind of statement it is: when GIA's article cites that ratio, it explicitly attributes it to the personal view of trade professionals, not to a GIA grading standard.
Context: GIA's "Colored Gemstone Value Factors" presents "colour is about 60%, origin can affect about 15%" as the personal opinion of interviewed trade professionals.
So whenever this site cites such ratios, it labels them "trade opinion, not a standard" and never derives a price from them.
What this site will and will not say
- No price ranges, no promises of appreciation, no investment advice;
- No 4Cs for coloured stones (the 4Cs are a diamond system);
- No "material A is always worth more than material B" — value depends on the specific quality, treatment status and market, not on the material type;
- Quality statements answer only "what the market looks for" (colour saturation, clarity, cut proportions), never "what it is worth";
- A quality description does not grade any specific specimen — that needs a report from a qualified laboratory on that specimen; see laboratory testing and report wording.
Conclusion of this page
Quality factors describe which characteristics the market generally prizes; they are not a grade for any specific specimen and are neither price nor investment advice. The 4Cs system applies to diamonds and is not transferable to coloured stones.