Colour

The majority of diamonds range from colourless to tones of yellow or brown. One of the four C€™s, colour, is an important consideration in determining the value of a diamond. The most desirable and valuable stones are at either end of the colour grading scale. This applies to those diamonds, which are colourless or near-colourless, and those that are intense fancy colours. Diamonds that are colourless as well as those, which are intense, fancy colours occur only rarely in nature. As a consequence they are more highly prized and command a higher price.

Diamond colour is mostly the result of natural processes during their formation. However, today, diamond colour can be manufactured synthetically or colour can be created or removed, from natural diamonds by artificial means.

There are many causes of colour in natural coloured diamonds. Inclusions, trace impurities and structural defects are all known to cause different colours. The mechanisms by which this occurs are not visibly apparent and these scientific processes will be explored later. They€™re often conclusive proof of their natural origin.

Only a very small percentage of diamonds (approximately 1 in 10 000 carats) found are coloured, known as fancy colours €“ pink, blue, green, grey, black, yellow, and even more rarely orange and violet.

Most diamonds seen in store windows appear to be colourless or white, but there are, in fact, slight subtleties of colour difference, which can greatly affect the value.
An easy way to visualize this concept is if you think of a glass of tap water, clear and completely colourless. By adding a drop of dye to the water, it may still appear colourless to the eye, but in fact, it is actually a shade darker than before. If we continue to add more drops of dye, the colour concentration increases gradually until colour is completely saturated. This symbolizes the very gradual scale of increasing body colour in diamonds. The colour difference between diamonds may be very slight, but the impact on their value can be enormous. This is why it is paramount, that diamonds are graded by an experienced diamond professional, in controlled conditions.

In the 1950€™s GIA developed the colour grading scale, using alphabetical letters, D to Z. Assigning D as colourless ranging to Z as light yellow or brown. Use of the GIA grading scale spread worldwide and has since become a universal means of classifying colour. For this range of diamonds, colour grading is an assessment of the relative absence of colour. Prior to this development, colour of a diamond was either described by the mine from which it originated, as in Wesselton and Cape, or by terms used by cutters, such as Fine White or Commercial White. It was decided to start the GIA grading scale at D so as to avoid confusion with any previous systems.

D to Z gradings do not denote an actual colour but instead a range of tones of colour and its saturation or intensity. Colour grading is a measure of the depth of colour by means of comparison.

It is important to note that each letter represents a range of colour intensity that merges into next. To the untrained eye, there may be no distinguishable visual difference between two diamonds, such as, a low F colour and a high G colour. However, this can greatly affect the price.

THE DIAMOND GRADING TECHNIQUE

GIA was the first to assemble a master set of colour grading stones to act as the borderline between two assigned colours. Any accurate master set needs to calibrated to a comparison set from a well-recognized certification laboratory, such as GIA or HRD. It is important to use diamonds rather than simulants as a master stone set, because the colour of diamond is stable and light reacts differently in simulants. Using simulants can confuse the eye, resulting in an incorrect reading.

The environment in which diamonds are viewed must be as neutral as possible. They should be assessed against a non-reflective white background using diffuse white daylight-equivalent conditions. The circumstances for grading diamonds should be constant and stable in order to achieve accurate comparisons.

Accurate colour grading cannot be done with stones mounted in jewellery. The metal can greatly mask or influence the true colour grade. For mounted stones colour can only be estimated.

To colour grade, the stone and master comparisons must be clean, free of any grease or residue that can possibly alter colour. The diamonds are put in a white plastic or paper-grading tray in order to eliminate the effects of the surroundings. The stones are placed face down in the tray, spaced apart so colour doesn€™t impart on adjacent stones. The eye should focus on the pavilion region to assess depth of colour. Subtle differences are usually more visible by looking through the pavilion and the eye is not distracted by the twinkling of facets. The stone needs to be viewed on both the left and right sides of the master. This is because a diamond may appear lighter on one side of the masterstone than the other. A larger stone may appear to hold more colour than a smaller stone of the same colour grade merely due to its volume. Conversely, stones less than 0.25ct are difficult to colour grade because of their small size.

Once its position is established between the borderline masters, a colour grade can be assigned. The most important factor for determining accurate colour grading is experience. Even highly trained graders do not rely on €œcolour memory€; the human mind cannot accurately remember slight differences. In respectable gem laboratories, more than one grader examines the diamond to insure consistency of colour grading.

FLUORESCENCE

It is still a mystery as to what causes some diamonds to fluoresce while others don€™t. Although it is believed to be associated with the internal atomic structure, research into this is still ongoing. It has been estimated that up to 35% of diamonds are fluorescent to some degree. Fluorescence is the emission of light only visible when exposed to ultra-violet radiation. It can only be assessed by viewing the diamond under long-wave ultraviolet light. The fluorescence of a stone is usually not apparent in normal lighting conditions, that is, fluorescent light, sunlight and incandescent light. In most viewing circumstances, fluorescence is not distinguishable and does not affect colour appearance or transparency. Though a stone may fluoresce under ultra-violet light, in normal lighting conditions it is often difficult for even a highly trained diamond grader to detect.