It appears from the articles and public presentations in Basel and Hong Kong that the GIA system will use averaged crown and pavilion angle data. Lower symmetry grades will lead to lessor overall cut grades. There are at least three shortcomings from this approach.
Firstly consider the idea that there can be ‘sweet spots’ within a grade. If all the measured proportions are such that the stone is well within grade boundaries, then symmetry deviations may have little impact on the diamonds appearance. The GIA authors have qualified this by the fair claim that a diamond in the top grade should also meet certain crafting quality standards. But consider a diamond that is near the boundary of a proportion grade that has certain types of symmetry deviations, like for example, a slight squarish out of roundness that results in the combined effect of steeper crown facets directly over deeper pavilion angles. The additive effect of this type of small symmetry deviation on the ray paths in a diamond can result in far greater leakage and reduced light return. AGS would be able to downgrade such stones because of their combined use of parametric and direct determination using their 3 colored ASET scope.
Secondly, the GIA article mentioned that diamonds that exhibited excellent symmetry with Ideal-Scope or Hearts and Arrows viewers were not rated any higher by observers than those with lessor patterns. I suspect that their study only focused on diamonds within very good proportions, and in this respect, Peter Yantzer and I have never subscribed to the idea that Hearts and Arrows diamonds offer any better appearance to the naked eye than well cut diamonds with small symmetry deviations. Had GIA tested diamonds with the ideal-scope with the purpose that it is designed to be used for, rather than simply as a gauge of symmetry, I feel they would have discovered benefits form its usage and weaknesses in their current symmetry grading method.
 |
| These stones have a 1% out of round squarish profile, yet they would be graded as having excellent symmetry by all labs. The stone on the left - four of the eight sets of main facets have crown and pavilion angles of less than 34.9° and 40.9°, while the section that is leaking with a less well defined star, has crown and pavilion angles of 35.1° and 41.2°. The HCA scores for each set of proportions are 1.7 and 3.4 respectively. The stone on the right is the same model with a one half degree shallower pavilion; it is in a “sweet spot” with HCA scores of 0.7 and 1.4 respectively. |
Thirdly, another disadvantage of a parametric system is that it takes no account of a diamonds size. GIA will give the same grade for different sized round brilliants with equal proportions[ii] . Tom Moses acknowledged that a 1/3rd carat diamond with equivalent parameters would have a different appearance to a 5ct diamond. Better symmetry in a well cut 1/3rd carat diamond aids in producing easily discernable firey and contrasting facets. But in larger diamonds, of say 5 carat or more, some types of symmetry deviations can actually improve a diamonds appearance[iii] by increasing the frequency of flashes from an observably larger number of virtual facets, even though both stones have the same number of facets.
The current methods for grading symmetry has evolved from the basis of the capacity to assess or detect deviations. These systems can be shown to be inadequate for the current task.
Rounding errors
As noted, the GIA has applied a rounding of parameters, presumably because of the inherent accuracy of existing scanners. Rounding introduces errors. Accurate measurements of azimuth are also critical for parametric grading when considering the Indexing; Sarin and Ogi scanners appear to have limitations in this area.
Upper and Lower Girdle Facet Indexing
GIA published an article 13th Feb 2004 discussing azimuth and angular adjustments or minor facets that are achieved with the aid of ‘indexing’. The accounts of the affect of these polishing practices on diamond appearance ran counter to the widely held opinions of others in this field. For instance diamonds that have painted or built up upper girdle facets like EightStar diamonds® were said to show “little contrast except for the dark appearance of the pavilion mains beneath the table”. The current article makes no mention of how painted or dug out facets will be interpreted in a new grading system, nor of how these azimuth shifts could be detected with adequate accuracy.
[ii] I asked about this on behalf of Sergey Sivovolenko from OctoNus at the GIA Basel Gemfest 2004 presentation. Tom Moses replied that they would give the same grade to a 0.30ct diamond and a 5ct diamond if all proportions etc were the same.
[iii] Personal communication with Pol Van der Steen (DiamCad, Belgium) who is a specialist manufacturer of very large diamonds.