The gem industry is yet to establish a single, universally accepted Coloured Gemstone Grading System for coloured stones. However, buyers and professionals can rely on several respected coloured gemstone grading methodologies at the moment.
The Gemmological Institute of America (GIA) has developed a comprehensive-coloured stone description system that serves as a foundation for much of the industry’s-coloured gemstone-based communication. This system evaluates colour through three interconnected properties: Hue, Tone, and Saturation.
Hue is the basic colour impression, the first perception when viewing a gem. GIA's system uses 31 hue names based on the RGB model to describe gemstone colours. While a sapphire might be described simply as "blue," more precise descriptions capture combination hues like "violet blue" or "green- blue."
Tone describes how light or dark a stone's colour appears. GIA's tone scale ranges from 0 (colourless/white) to 10 (black), with key reference points at 3 (Light), 5 (Medium), and 7 (Dark). For blue sapphires, the ideal tone typically falls between 5 and 6, which is medium to medium-dark, providing the optimal balance of colour visibility and depth.
Saturation measures the intensity or purity of colour. The saturation scale ranges from 1 (greyish/brownish—weak) through 6 (Vivid—the strongest saturation). In warm colours, lower saturation appears brownish; in cool colours like blue sapphire, it appears greyish. The most valuable blue sapphires achieve a saturation grade of 5 (Strong) to 6 (Vivid) combined with optimal tone.
It is important to note that GIA does not formally "grade" coloured stones in the same way it grades diamonds. GIA's Gemmological Identification Reports for coloured stones assess characteristics including weight, measurements, shape, cutting style, and colour description. They determine whether a stone is natural or laboratory-grown, identify detectable treatments, and can provide an opinion on geographic origin upon request.
GIA has introduced a clarity type classification system that acknowledges different gem varieties naturally form with different inclusion characteristics. The system classifies gems into three types:
Type I gems including Aquamarine, Citrine, or Topaz, typically grow with few inclusions and are usually "eye clean”.
Type II gems such as Sapphires, Ruby, and Spinel commonly contain inclusions.
Type III gems like emerald, red tourmaline are almost always included.
For Type II gems like sapphires, clarity is assessed without magnification at a distance of approximately 10-15 centimetres, evaluating whether inclusions are visible to the unaided eye.
American Gemmological Laboratories (AGL) has developed its own comprehensive grading system using numerical scales for colour evaluation. Under the AGL system, colour grades are expressed as decimals (e.g., 3.5), where lower numbers indicate more desirable colours.
A 3.5 colour grade on an AGL report is comparable to a D colour diamond on a GIA diamond report, representing near-ideal colour.
AGL's system places particular emphasis on primary colour percentage, with stones showing higher percentages of their primary hue receiving better grades.
Some markets use informal "AAA" grading systems (AAAA, AAA, AA, A), particularly for tanzanite and occasionally other coloured gems. However, this system lacks universal standardisation and varies significantly between sellers, making it less reliable for comparison purposes.
