The "cape series" as the basis of color-grading is subdivided into eight color grades. It begins with absolute colorlessness, followed by zones with slightly increasing yellow saturation down to the lower color-grades which reveal clear yellow coloration (Fig 1).
|Exceptional white||Rare white||white||Slightly tinted white||Tinted white||Tinted|
Fig 1 color grading is based on the observation of the intensity of an increasing yellow saturation
The transitions are "fluid" and the colors not clearly defined, which, especially the higher color stages of "exceptional white" and "rare white", can lead to difficulties in the beginning. The difference here lies rather in the degree of transparency than in the color suffusion. Only much practice and experience can produce in the fullness of time the necessary certainty to classify a diamond correctly into the right color-grade.
Perception of color in smaller diamonds is more difficult then in larger stones, as the small surface area of the facets, combined with the dispersion, render the body color of the stone less clearly visible than in the larger-surfaced facets in diamonds of over half a carat.
In two diamonds of different sizes cut from a like-colored rough stone the larger stone gives an impression of more intense color, for the absorptive power becomes greater with the increasingly long. path of. the light rays in the diamond. The following classification may be given for determining the color of diamonds according to their size, when the stones are viewed through the table.
An accurate color-grading can be achieved only with reference to the following factors:
a suitable light source;
an ideal color comparison set of master diamonds;
the environment of the color-grading;
the experience and grading ability of the grader;
the position of ,the brilliant during the grading.
According to the traditional method diamonds were examined in a North light (or South light in the Southern hemisphere) in a folded white paper, and viewed through the table, sideways above the girdle, or from below the culet. By this means any difference in color from that of the white paper was estimated. Apart from the fact that this method permitted only an approximate judgment as to the quality of color, the observer was dependent on the change of daylight from light to dark, from a reddish tint in the morning through bluish at midday, to red in the late afternoon and evening. Especially in the winter' months, color-grading was restricted in time to the few light hours of the day. And, just as smoke, dust and mist in the atmosphere could have unfavorable effects, so, too, the immediate environment prevailing at the time tended to exert its influence on the color determination. For yellow or blue walls, doors and curtains, or the clothes of the grader cast reflected light on the stone and affect the test either positively or negatively.
For these reasons artificial light-sources were substituted. Their primary disadvantage was that the lamps used varied from dealer to dealer; almost ever: jeweler or stone merchant used a different type of light and accordingly, every diamond looked different in color. A further fault lay in using too concentrate and too bright a level of illumination, which masked the actual body color 0 the strongly glistening and scintillating diamond.
Moreover, there was no means of ascertaining the position of the
relation to the yellow seals, as this is only possible by comparison with diamonds whose color values have been measured objectively. Thus unexceptionable color-grading was not possible. The spectral distribution of the light used should correspond as nearly as possible to a "North light", that is if it should approximate to daylight, and at the same time be diffuse and not too intense. As these natural conditions are rarely achieved, the use of artificial light sources has prevailed in the practice of diamond grading, especially of light tubes of color temperature 5,000-5,500° Kelvin (0 55-0 65), which are also recommended by the CIBJO, GIA, and the 10C. They produce a light similar to daylight with an equivalent spectral distribution.
The light tubes of such lamps are clad on their inner side with a white coating, which in the course of time turns yellow and can discolor the light. When use of the lamp is frequent, the tubes should therefore be changed from time to time.
|Scan. D. N.||Comparison
Brilliants as the Basis of color-Grading