The GIA hue code is a notation system used by the Gemological Institute of America in its Coloured Stone Reports to identify the dominant colour of a faceted gem on a 31-position colour wheel. The system is one component of the GIA Coloured Stone Grading System, which also includes tone and saturation grades.

The 31 positions

The hue wheel positions are notated by letter codes representing the principal hue colours and their transitions. The principal hues are red (R), orange (O), yellow (Y), green (G), blue (B), violet (V), and purple (P). Intermediate positions are denoted by combined codes in which the modifying hue precedes the dominant: a stone whose hue lies between red and orange but closer to red would be coded oR (orange-red, with the lowercase letter indicating the modifier). A stone closer to orange would be coded ROr (red-orange, where the dominant hue is the second letter of the pair).

The full set of 31 positions provides finer division than the colour names alone would permit, allowing for the gradations between adjacent principal hues to be specified. The system is descriptive of the dominant face-up colour as observed under standardised lighting conditions in the GIA gemological laboratory.

Use in reports

The hue code is paired with a tone grade (1 to 10, where 1 is colourless and 10 is black) and a saturation grade (1 to 6, where 1 is greyish or brownish and 6 is vivid) to produce a full colour designation in the form, for example, slbB 5/4: slightly bluish blue, medium tone, strong saturation. The notation is used internally in the laboratory to record colour observations and is included on certain GIA Coloured Stone Reports as an objective record of the stone's colour at the time of grading.

The system is calibrated for daylight or a controlled equivalent light source (typically a 6,500 K fluorescent), and the grading is performed face-up against a neutral grey background by trained graders. The system does not capture pleochroism, colour shift under different lighting, or the brilliant return of light that the trade describes as life. Those characteristics are recorded separately on the report or are not formally graded.