3.1 Determination of Colour Grades

3.1. DETERMINATION OF COLOUR GRADES

The diamond colours can be divided in two main groups:

• colourless to light yellow diamonds (or equivalent)diamonds with a yellow hue, possibly with a faint brownish, grayish or
  greenish deviation
• coloured diamondsall diamonds darker than light yellow (or equivalent) and
  all diamonds with another colour hue

3.1.1. COLOURLESS TO LIGHT YELLOW DIAMONDS (OR EQUIVALENT)

3.1.1.1. COLOUR GRADES

These colours are subdivided into 10 colour grades, called the "International
Colour Grading Scale" (see colour comparison table 3.1.1.2.)

3.1.1.2. INTERNATIONAL COLOUR GRADING SCALE

3.1.1.3. DETERMINATION OF THE COLOUR

The colours must be determined by comparison with masterstones.

After a complete validation and on the base of a comparison between the results of a measuring instrument with those of a grader, colour measuring equipment can be used for grading.

3.1.1.4. POSITION OF THE MASTERSTONES

The colour grades are fixed by a series of masterstones, which, as a definition, fix the lower limit of each colour grade. An international reference-series has been composed by representatives of the WFDB, the IDMA and CIBJO.

3.1.1.5. SELECTION OF MASTERSTONES

A series of valid masterstones must be selected by direct comparison with the international reference series.

The masterstones must comply with the following requirements:

• brilliant-shapes only
• weight: larger than 0.70 ct
•  proportions: vg

table diameter: 60 to 66%
crown height: 11 to 14%
girdle thickness: smaller than 3%
pavilion depth: 42 to 45%

• clarity: vs2 or better, free of colored or black inclusions and structure phenomena causing colour effects
•  girdles: bruted, faceted or polished girdles (the nature of the girdles must be the same within 1 set of masterstones)
• fluorescence: nil or slight
• colour: yellow hues only

3.1.1.6. CONDITIONS

The colour comparison with masterstones is performed under normalized artificial light, equivalent to northern daylight. It is recommended that the characteristics of the artificial light source used, approximate the C.I.E.-standard illuminant D65 (C.I.E. = Commission Internationale de I'Eclairage / International Commission on Illumination).

The masterstones and the diamond to be graded are placed on a V-shaped white support (e.g. a folded white paper) and observed from a direction, perpendicular to the pavilion facets.

3.1.1.7. GRADING

The comparison with the masterstones is made as follows: if no difference in colour is observed between the diamond to be graded and a specific reference-diamond, then the colour of the latter is conclusive.

If the colour of the diamond to be graded lies between those of two reference-diamonds, then the colour of the darkest one is conclusive.

3.1.1.8. EQUIVALENT COLOUR

Diamonds of equivalent colour are compared with the same masterstones and in the same manner as diamonds with light-yellow colour.

In case of a clearly visible distinction, this has to be mentioned together with the colour grade.

3.1.2. COLOURED DIAMONDS

3.1.2.1. Determination

The colour determination is performed by comparison with colour standards.

3.1.2.2. Description

The following characteristics are described:

• colour hue
• colour saturation and lightness
• colour origin

3.1.2.3. Denomination

Depending on the above mentioned characteristics of the colour, the term "fancy" may precede the colour description.