No two diamonds are exactly alike. However when it comes to color it is obvious that the colorless to light yellow diamonds form a much more homogeneous category than the colored diamonds. This last group displays a bewildering variety of colors, and small nuances can make an important difference.
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Colors can vary considerably depending on the circumstances in which the stone is viewed. Diamonds appearing brownish-pink in daylight can be miraculously transformed into wonderful reddish diamonds under a more yellow light source. Other diamonds, aptly called “chameleon-type,” actually change color under the influence of different circumstances. Some of these take their time in returning to their original hue, so that it is possible to actually see them change gradually over a period of time after exposure to different light. For instance, some pink diamonds may suddenly turn brown when illuminated with ultraviolet (UV) light, only to return to their original color after being removed from UV light. Other diamonds can temporally change color when kept in complete darkness for a while, or when they are heated.While judging “normal” colors is mainly a matter of comparison with master stones to see whether diamonds is darker or lighter than the test stones, evaluating color diamonds is a much more complex problem. The description of colored diamonds proves to be a real challenge to a diamond-grading lab. Colorimetry is in itself a complex matter, and the fact that diamonds have very specific physical properties does not make matters any easier.
A Lab must therefore develop a system for diamond color grading that takes into account all possible problems such a system must meet certain requirements:
1. It must be scientifically sound and based on objective standards
2. The nomenclature used must be clear to the client. Scientific jargon must be translated into terms intelligible not only to the professional but also to the consumer;
3. The system must be transparent and not unnecessarily complicated;
4. Consistency must be sufficiently guaranteed;
5. And the system must answer the needs of the market.
In designing the Antwerp diamonds high council (HRD) fancy color system considerably effort was made to combine all these factors. It goes without saying that maximum consistency is the primary concern. The other issues demand a compromise between scientific accuracy and complexity on the one hand, and transparency and accessibility on the other hand.
The first step was to look for a reference standard to be used as a basis. Extensive tests have proven that color description based purely on visual observation without reference material is not reliable. Therefore, the Munsell Book of Color was chosen to serve as the reference standard for the HRD system. From HRD’s point of view, an extra recommendation is that the firm Macbeth, editor of Munsell Book of Color, is certified according to Internationals Organizations for Standardizations (ISO) 9001 standards and that all Munsell color samples are precisely measured spectrophotometrically to ensure that strict manufacturing tolerances are met. All Munsell standard measurements are furthermore traceable to NIST (National Institute for Standards and Technology). CIE data (International Commission on Illumination) are also available.
The Munsell Book of Color is a collection of color standards, in the shape of removable color chips, catalogued according to the three color parameters - hue, chroma, and value. For the description of the three parameters, Munsell has given each parameter a numerical scale. Each color chip in the book can be identified by its unique combination of numbers referring to the three scales. Each color card can therefore be defined by the so-called “Munsell Notation” - HV/C (Hue Value/Chroma).
Hue is the parameter distinguishing between red, yellow, green, blue and purple. These colors fade into one another. Black, gray, and white do not have “hue”; they are called “neutral colors.” All other colors that do have hue are called “chromatic colors.”
Value is a parameter to distinguish between light and dark colors. The scale runs from 0 (black) to 10 (white). The scale can be used both for neutral and chromatic colors.
Chroma expresses the measure of saturation of the color. It defines whether it contains a “weak” (i.e., low chroma) or “intense” (i.e., high chroma) color. The scale starts from 0 and can, for normal materials, range to approximately 18. This system is (relatively easily applied to color diamond grading. The diamond is compared to the Munsell color cards, and the card that resembles the color of the stone the closest is determined. Once the right color card has been selected, the color can be named accordingly.
It is possible to describe colors on various levels of accuracy. This system couples a certain name to each of the Munsell color cards. This name consists of various elements. First of all, there is a description of the hue, giving names such as yellow or yellow-green. Secondly, each color chip also gets a prefix, based on the combination of the value and the chroma. A color with a certain chroma can have a high or low value and vice versa. Prefix and color hue together (e.g., moderate purplish-red) should give quite a good idea of the color, even without seeing the diamond and without having access to the Munsell book.
The color hues give a good indication of the color, and the prefixes used by the National Bureau of Standards (NBS) are very detailed, but for noncolor specialists they can be confusing. Therefore, HRD has simplified and adapted the NBS system to make it more suitable for the description of diamond color. It was certainly not the intention to suggest that this is an improvement on the NBS system, which, on the whole, is very sound and logical. On the contrary, it is in fact a simplification dictated by the needs of the specific application in question, which does not allow the system to be used in its entirety.
In practice, colored diamonds are often referred to as “fancy colors.” However, the undiscriminating use of a term renders it useless and it is therefore not advisable to simply use this for every color outside the colorless-to-slightly-yellow range. Most people in the diamond trade agree that “fancy” should be reserved for especially “beautiful” and “attractive” colored diamonds. For a lab, of course, this is not a workable definition as it is based on purely subjective taste. Therefore, a definition was drawn up combining requirements as to color hue and intensity - the diamonds must have a pure color and the intensity must fulfill certain criteria, dependent on the hue.
In an attempt towards classification, diamonds can be divided into three large groups - diamonds with a yellow color hue, diamonds with a brown/gray color hue and diamonds with a different color hue. This can be schematically represented as demonstrated below. Only the diamonds falling in the shaded zones can be called “fancy.”
It goes without saying that this grading system is supported by the use of master stones defining certain limits. The descriptions are, after all, open to interpretation, and comparing diamonds with reference stones is still the best method to ensure consistency in grading.
Talking about consistency and repeatability, it is important to stress that the final decision is always based on the judgment of several graders (as are all decisions in the HRD lab, whether it concerns color, clarity or cut).
The grading system described here has been in use at HRD for many years, and has amply proved to be practical and serviceable. Nevertheless, constant efforts are made to optimize the techniques, and if possibilities for improvement occur these are integrated in the system.
Note that all the preceding remarks apply to natural diamonds with a natural color origin. Diamonds owing their color to artificial treatment or synthetic diamonds are, predefinition, not graded by HRD.
PREFIXES USED BY HRD:
Faint: The color hue cannot be perceived through the crown side of the stone, and is only faintly noticeable through the pavilion side. For example, faint pink.
Light: The color hue can be perceived through the crown side, and is clearly noticeable through the pavilion side. For example, light blue.
No Prefix: The color hue can be clearly perceived through the crown side and the pavilion side of the stone. For example, blue.
Intense: The color hue is very noticeable through the crown side. Saturated color with normal to high value. For example, intense yellow.
Dark: The color hue is very noticeable through the crown side. Saturated color with low value. For example, dark yellowish brown.
Translucent: “Translucent” in this context is used according to the scientific definition, i.e. “transmitting light but not transparent.” In other words, translucent stones have a reduced transparency. The intensity is not further specified. For example, translucent white.
