Because gem colour descriptions are so diverse and subject to individual perception, the Gemological Institute of America® (GIA) developed the Coloured Stone Grading System. It helps one ascertain a stone's colour from the ‘most preferred’ to the ‘least preferred’ available for its species. In this course, we will use the Gemewizard system, which is the digital implementation of this industry-standard colour description system.

To describe a gem's colour using the system, one will have to answer three questions:

• What is the gem's colour (Hue)?
• How light or dark is the gem (Tone)?
• How strong is the gem's colour (Saturation)?

Hue :

Hue is the first impression of an object's basic colour. The colour may be made up purely of one of the basic hues, such as red, orange, yellow, green, blue, violet and purple, or a combination of hues.

The Gemewizard (GW) system uses 31 master hue names, which include the basic hue names (red, orange, yellow etc.) and the combined hue names with secondary or modifier hues.

Abbreviated notation is used to describe the colour, wherein the dominant (or stronger) hue is capitalized. For example:

• red modified by a slight amount of purple: slpR (slightly purplish Red)
• if the purple modifier is strong: stpR (strongly purplish red)
• if there are equal amounts of purple and red: RP/PR (Red-Purple or Purple-Red)
• if purple dominates: rP (reddish Purple)

Tone:

Tone is the degree of darkness or lightness of a colour. The system divides tone into 11 levels, ranging from colourless (transparent) or white (opaque) - designated as 0 - through increasingly darker greys to black - designated as 10.

In practice, only levels 2 (very light) through 8 (very dark) apply to transparent coloured gemstones. Tones that are any darker or any lighter are almost impossible for the human eye to detect. Hence the GW system describes tones between levels 2 and 8.

The three standard tone levels are 3 (light), 5 (medium) and 7 (dark).

Saturation:

Saturation is a colour's strength or intensity. It is assessed on a six-level scale (1 through 6).

If the hue has any brownish or greyish components, the saturation level is 3 or lower. If brown or grey appear distinctly, saturation is at levels 1 or 2. If one can only see the slightest hint of brown or grey, then saturation is around 3. If there is no trace of brown or grey, saturation is 4 or higher. Saturation rating of 4 or higher (with all other parameters being equal) makes an attractive coloured gemstone.

For most coloured stones, valuable colours tend to fall within medium-light to medium-dark tone (4, 5, 6) and higher saturation (4, 5, 6). Therefore, each Gemewizard colour bears a three-digit code, representing the hue, tone and saturation values, which equates to the GIA® colour description codes.

For example: a gem's colour of R 6/4 (or 1-6-4) can be described as ‘medium dark, moderately strong, Red’.

Source: Gemstone Color Academy - www.gemcoloracademy.com