- Table off-center
- Culet off-center
- Table/culet alignment
- Table size variation
- Girdle thickness variation
- Crown height variation
- Crown angle variation
- Pavilion depth variation
- Pavilion angle variation
Physical Symmetry vs Optical Symmetry
One would come to wrong conclusion that if all 10 physical symmetry parameters above are perfect that the diamond would also appear optically symmetrical. The reason this is not correct is because there is another plane, the azimuth, where symmetry defects can occur.
A shift in the azimuth, can be visualised as a twisting the facets. A diamond that is twisted can pass the tests above yet appear asymmetrical when viewed through a hearts & arrows viewer.
This twisting is known as yaw and it’s a defect when it occurs because the cutter at the grinding wheel is forced to take material off unevenly. Sometimes it is done on purpose to hide what otherwise would be defects in the physical symmetry so cutters refer to this technique as ‘cheating’ the lab test.
Physical Symmetry Limits
The table below shows the limits used by the GIA for the 10 symmetry parameters for the excellent symmetry grade:
|Out-of-round %||0 – 0.9|
|Table off-center %||0 – 0.6|
|Culet off-center %||0 – 0.6|
|Table/culet alignment %||0 – 0.9|
|Crown height variation %||0 – 1.2|
|Crown angle variation (o)||0 – 1.2|
|Pavilion depth variation %||0 – 1.2|
|Pavilion angle variation (o)||0 – 0.9|
|Girdle thickness variation %||0 – 1.2|
|Table size variation %||0 – 1.2|
This table has potential to be confusing so we will go through each of the parameters. We have previously discussed the problem of this system regarding the crown and pavilion angles in our "diamond grading" tutorial so we will now focus on the rest of the parameters.
According to this table, in order to achieve a GIA graded excellent stone, the maximum length and width of a diamond must be within 1% of each other. For a 1 carat stone, we expect the diameter to be 6.5mm. This means that a diamond can have a variance up to 0.06mm. With measurement limits at a margin of error of 0.02mm, this would mean that the variance can be up to 0.08mm. An out-of-round diamond can have an effect on all the other facets and therefore its recommend finding diamonds with a length to width variance of 0.02mm or below.
Bear in mind that table off-center is measured from the table edge to the center of the table. A diamond will receive an excellent grade if the maximum table off-center variance is 0.6%, that does not mean that the variance is 0.6% in terms of a linear measurement. This percentage relates to the table % in relation to the diameter, as shown in lab reports. This is roughly 0.04mm for a 1ct stone (0.06mm when you account for margin of error). This means the table can be off-centered up to 1% of the diameter and still be considered excellent.
If a diamond has a 58% table, this suggests that a diamond can receive an excellent grade if on one side it has the effect of a 59% table, and on the other side the effect of a 57% table. If you account for both sides of the diamond, the variance from the 58% table stated on a lab report is actually doubled to 2% of the diameter of the diamond. Remember, this is different from the table size grading because that is based on a reference point from the center of the table facet.
Because of these problems with measurement, the GIA recommends cutters to aim for limits 20% below the graded limits to be safe. The reason is that on top of these limits, the GIA will still visually assess the diamond and penalise the diamond accordingly. Therefore, the example in the last tutorial of a hypothetical stray crown angle is not likely to occur in practice. However, this raises other concerns because whenever you have humans making an assessment, there is always the scope for human error to creep in. Therefore, it is imperative that you understand what the labs grade as symmetrical so that when you assess your diamond you will be able to check to see that the diamond is graded correctly.
All of the above symmetry features are considered to be proportion-related features. There is still one feature that I have not mentioned so far and that is a ‘wavy girdle’. This is exactly how it sounds and it means that all the angles can be correct and the girdle thickness is correct but it is not straight. It occurs when crown-only painting is compensated with pavilion-only digging; more on this in the upcoming advanced tutorials.
The other parts of lab-graded symmetry are facet-related symmetry features. According to the GIA article on symmetry, there are the following parameters.
- Extra Facets
- Missing Facets
- Misalignment of Facets
- Misshapen Facets
- Non-octagonal table
Misalignment and Non-Pointing
Misalignment and non-pointing are what are considered meet-point-symmetry. This simply means that the corners of all the facets of the diamond should come to a point. This is characteristic of diamond as a material because synthetics are cut so that the points don’t meet on purpose to enhance their light performance. A diamond’s high refractive index allows the cutter to make sharp meet-points without negatively impacting light performance.
Misshapen means that not all the facets are the same size. For example, some of the arrows of the diamond are thicker than the others. This would indicate that the pavilion mains are not the same size.
A natural is simply a piece of unpolished rough that is left on the diamond. An external natural will be indicated green on a clarity plot to distinguish it from an indented natural, which will be red on a clarity plot. Naturals are usually a polishing feature but if it is so large that it affects the out-of-roundness of the diamond, then it can impact symmetry as well.
The polish of a diamond directly affects it’s luster. Luster refers to how much a diamond shines and not how it sparkles. Many times most have seen people in the trade recommending that consumers stick to excellent and very good polishing grades. A diamond with a polish grade of excellent is not necessarily void of minute polishing features. However, a diamond with minute polishing features cannot receive an FL clarity grade. Even in IF quality stones, you will notice a comment that states that minor details of polish are not shown.
Is a very good grade a money-saving bargain?
A very good polish grade means that the polishing features can be seen under 10x magnification.
This is what the GIA says about a very good grade:
“Some typical features that would establish a Very Good category include several pits or nicks, a few small areas of abrasion, a limited extent of moderate transparent polish lines, a small area with faint white polish lines, several faint scratches or a few heavier white scratches, faint lizard skin, or a small area of very faint burn marking.”
Most do not feel comfortable recommending a very good polish grade for VS quality stones or above. However, a very good polish grade could be a money-saver if you are looking for an SI clarity diamond.