Testing and grading coloured diamonds
Coloured diamonds are unique and more challenging to grade than their colourless counterparts, as no two are alike. Each stone may have secondary hues besides the primary colour at varying intensities. They are graded by comparing to a master set of the same hue, or with non-transparent Munsell chips of similar hue, tone, and saturation. Few labs have a comprehensive suite of some colours. Even so, colour grading is inherently subjective without the use of spectrometers, which, as mentioned earlier, can measure colour more precisely. This method, however, has traditionally not been used by most labs, as it is more time-consuming, both in application and interpretation of the results. At our lab, gemmologists employ this method and indicate spectra results on diamond reports. While 3-D colour grading is possible, educating gemmologists, traders, and eventually consumers is necessary to understand how these numbers translate into a fancy colour grade system, such as GIA’s or Hofer’s.
Since values are based on grades, a report from a reputable gem laboratory should accompany any important coloured diamond weighing more than .50 carats to verify it as natural. The issue of testing and grading is three-fold:
Identification and colour origin
Testing with standard and advanced instruments is critical to properly identify and assign colour origin (i.e. natural, treated, or synthetic). I had the opportunity to study and certify part of a collection of ‘chameleon’ fancies. When heated to approximately 120 C, they exhibited temporary colour changes from olive to orangey yellow and some continued to do so after prolonged storage in the dark. This phenomenon is rare and increased the stones’ value by 10 to 20 per cent compared to one of the same quality that does not change colour.
In his book Collecting and Classifying Coloured Diamonds, Hofer uses nature-based terms such as ‘lemon,’ ‘honey,’ ‘canary,’ ‘rose,’ and ‘olive’ among others to describe colour. Perhaps the best methodology is to combine a ‘dry’ colour grade, such as ‘Fancy deep grayish greenish yellow’—which is GIA’s widely accepted grade terminology for coloured diamonds—with an additional comment like ‘Known in the trade as olive colour’ and scientifically position this colour in a 3-D system (i.e. numerical value).
Repeatability refers to the ability of a single laboratory to examine a coloured diamond and then at some later date, re-examine the same stone and reach the same conclusion. Similarly, it is important the colour grade assigned by one lab can be verified when the stone is submitted to a second one for testing. This is known as ‘reproducibility.’ Currently, there is very little repeatability and reproducibility in coloured diamond grading. In short, the entire colour grading system is in need of an overhaul so that each colour grade is assigned more accurately using modern technology, and less subjectively (i.e. relying solely on master sets and the human eye).