The Art of Colour Grading

Bellerophon Gemlab Group

When we establish the colour of a Gemstone or compare the colours of two gemstones side-by side, several factors must be considered:  

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1. Use a consistent, standard source of light with known illumination characteristics.  

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2. The observation should take place in an appropriate surrounding environment that is neutral in its colour appearance.  

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3. A defined geometry should be used between the light source, the object, and the observer.  

4. If the Gemstone colour is to be compared to that of another gemstone, the latter should be a standard colour reference.  

5. Observations must be made by a person with normal colour vision. Because any of these factors can influence the visual perception of a gemstone colour, they all must be controlled if accurate and consistent results are to be obtained. 

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Colour Description of a Gemstone.

Colour is a continuum that can be defined and described in terms of three attributes:  

1. Hue, the attribute of colours that permits them to be classed as, for example, red, yellow, green, blue, or anything in between.

2. Tone, the relative impression of lightness to darkness of the colour.  (the white and black component of the colour)

3. Saturation, the strength or purity of the colour.  (the intensity of the hue)

The Colour Grading light boxes 

The most important characteristics are: the light source, the lamp type, the measured correlated colour temperature (a term used to describe the colour of a light source, such as yellow or blue light, figure 1), its illuminance (the amount of light  per unit area on a surface, multiplied by the spectral sensitivity of the eye), and a calculated colour rendering index (a measure of the degree to which the perceived colours of objects illuminated by a given light source conform to those of the same objects illuminated by a standard light source) and lastly the calculated index of metamerism (a measure of the degree to which pairs of colours with different spectral characteristics appear the same under a standard light source and different under a “test” light source). These characteristics all contribute to a person’s ability to distinguish colours while using the particular light source.

Figure 1: The important influence of the light temperature use in the colour grading light boxes.

Viewing Angle Geometry. A coloured gemstone has a number of colour differences, and these differences are often subtle. Therefore, both the position of the gemstone in the viewing box, and how the stone is held, are important. gemstone face up colour is evaluated most consistently when the light source is positioned directly above the gemstone, the gemstone being placed in a plastic white tray and/or held by tweezers. The distribution of the colour is evaluated in relation to stone total face up area.

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Main Colour. The Bellerophon Gemlab system describes a single colour as being “the main colour” of the gemstone as a whole (except in case of bi-coloured gemstone). We define this single colour as the overall colour sensation seen when the stone is viewed face-up. Obvious surface reflection and dispersion are not graded, while windowing (see through areas), and/or extinction areas are averaged with the main hue. To help determine the characteristic colour, the grader moves the gemstone slightly by rocking the tray. This process of moving the gemstone through a slight angle minimizes the effects of surface reflection, dispersion, windowing, and extinction. The “colour grade” that a coloured gemstone receives on a Bellerophon Gemlab laboratory report is a description of this characteristic colour using standardized terms.

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Colour Reference. Once the characteristic colour has been determined, it is categorized in colour space by means of a side-by-side comparison with two or more coloured gemstones reference. 

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Figure 2: Main hue colour reference.

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Figure 3: Bellerophon Gemlab uses a total of 15 basic hue names with the corresponding approximate colours

Division & Modification of Hue. There are one kind of changes that can occur to the 15 basic hues and their boundaries, as one “moves”through the Bellerophon terminology. The use of division terms as the saturation decreases. At low saturation levels, and at either light or dark tone levels, we recognize fewer hue names because there is a smaller number of discernible colours. Broader distinctions are made between stones in very dark and very light tones, and at low saturation levels. 

Figure 4: All hue is further divided by the combination of saturation and tone. Unfortunately for clarity and simplicity some colour defined as hue may be a different saturation and tonality of a main hue. Such as Pink being a lower saturation and lighter tone of Red, the same is true of Brown and Orange. Therefore, this figure does not hold true for all hue. There is no pastel Red and no Deep Pink…

Using Instruments to Measure Colour. Certain attributes of colour appearance can also be measured with instrumentation, such as a VIS spectrometer, spectrophotometer and/or a colorimeter. The colour space of Bellerophon Gemlab colour grades are ultimately defined in CIE LAB colour coordinates. Colour measurement may provide finer colour distinctions for some materials, questions have arisen regarding the consistency of such instruments for quantifying the colour appearance of faceted gemstones (Collins, 1984). The accuracy of such measurement may greatly be influenced by the shape, size, the path length of the light and the colour homogeneity of the stone. Bellerophon Gemlab always uses instruments to measure the colour of a gemstone, but ultimately the human eyes through the grader with the help of the CIELAB colour coordinates, references gemstones and controlled environment decide the grading. As deriving a consistent set of standards for both visual observation and instrumental measurement of colour in gems is a great challenge.

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Cutting Factors in Colour Grading. Cutting by the decision making of the axis (for pleochroic stone), shape, proportions, facettes sizes, facettes numbers, facettes angles, and polishes affect the behavior of the light inside the gemstone, therefore cutting may play an important part of the final colour grading of a gemstone. It is not uncommon for a colour grade of a gemstone after recut to be different by several order in the grading system. 

Figure 5: Bellerophon Gemlab Master Hue. The simplified 2D Colour wheel display most colour grade existing in the Bellerophon Gemlab colour grading system. It is a a level 3 simplified colour system with approximately ~90 colour grades available.