A master fancy colour set is a calibrated assemblage of reference diamonds representing specific fancy colour grades, used by gemmological laboratories to assess the hue, tone, and saturation of coloured diamonds. Unlike the D-to-Z master sets that anchor the colour grading of near-colourless diamonds, fancy colour master sets are organised by colour family (yellow, pink, blue, green, brown, grey) and within each family by saturation grade (Faint, Very Light, Light, Fancy Light, Fancy, Fancy Intense, Fancy Vivid, Fancy Deep, Fancy Dark). The grading of fancy colour is more subjective and instrumentally complex than D-to-Z grading, and master fancy colour sets are correspondingly less widely available outside the major gemmological laboratories.

The fancy colour grading scale

GIA's fancy colour grading system, introduced in its current form in the 1990s, evaluates coloured diamonds on three dimensions: hue (the basic colour, e.g. yellow, orange-yellow, brownish-yellow), tone (lightness or darkness on a scale from very light to very dark), and saturation (the strength or purity of the colour). The combination of these three dimensions is summarised in the saturation grade — Fancy Light through Fancy Vivid — that appears on the GIA fancy colour grading report. The scale is non-linear in the sense that the price increments between adjacent saturation grades are not uniform; the move from Fancy Intense to Fancy Vivid in particular often involves a substantial price step.

Master sets for major colour families

GIA maintains master fancy colour sets for the principal colour families that occur in the diamond trade: yellow (the most common fancy colour, with the largest set), pink (less common, with a smaller but still substantial set), blue (rare, with a small set), green (rare, similarly small), brown (common but commercially less valuable, often graded relative to a smaller reference set), and grey. Each family's master set covers the saturation range from Faint or Very Light through Fancy Vivid (or Fancy Deep / Fancy Dark for stones at the high-saturation end of the tonal range). The combined GIA master fancy colour set inventory is among the most valuable single collections of reference diamonds in the world.

Comparator vs. master

The technical distinction between a master and a comparator in fancy colour grading is sometimes used loosely. A master is a single reference stone that anchors a specific grade boundary; a comparator is a set of reference stones used in side-by-side comparison with an unknown. In practice the terms overlap, and the GIA-published terminology refers to the entire inventory as the master set. Working comparators in the laboratory are subsets of the master inventory selected for the colour family being graded.

Lighting and viewing

Fancy colour grading uses standardised viewing conditions different from those used in D-to-Z grading. Stones are viewed face-up rather than table-down, because the face-up colour is what the consumer sees in the mounted stone. Lighting is daylight-equivalent (typically 6500 K colour temperature), and the viewing background is neutral grey rather than white. The grader compares the face-up appearance of the unknown to the relevant masters in the appropriate colour family and assigns a grade based on the match.

Subjectivity and training

Fancy colour grading is subject to greater observer variation than D-to-Z grading because the colour space is multidimensional and the perceptual judgments are more complex. GIA fancy colour graders undergo extensive training and ongoing calibration, and major fancy colour grades are typically determined by consensus among multiple graders rather than by a single grader's judgment. The major laboratories also use spectrophotometric measurements to support visual grading, particularly for the more valuable higher-saturation stones.

Availability and cost

Master fancy colour sets are not generally available to the broader trade in the way that D-to-Z master sets are. The cost of assembling a credible fancy colour master set runs into the high six figures or seven figures (the comparator stones must themselves be fancy-coloured diamonds of known grade), and the demand outside major laboratories is limited. Most retail and trade operations that handle fancy colour diamonds rely on GIA or AGL grading reports rather than maintaining their own master sets.

In the trade

For dealers and collectors handling fancy colour diamonds, the GIA fancy colour grading report is the practical reference, with AGL and a small number of other laboratories providing alternative or supplementary grading. The standard published references are the GIA Coloured Diamond Grading course materials and the periodic articles in Gems & Gemology on coloured diamond grading methodology.

Further reading

• GIA — fancy colour diamond grading
• GIA Gems & Gemology — coloured diamond articles
• American Gemological Laboratories — fancy colour grading