Apart from the cut grades, grading labs also give grades for symmetry and polish. Both the symmetry and polish grades also have an influence on the overall cut grade. You can find the exact GIA’s guidelines of how the symmetry and polish of a diamond influence its cut grade here.
Which symmetry grade to choose?
The symmetry of a diamond refers to the symmetrical arrangement and even placement of the facets and to the exactness of the shape of a diamond. There are a handful of characteristics that are considered in the evaluation of the symmetry grade.
A diamond with an excellent cut has to have “Excellent” or at least “Very Good” symmetry characteristics. GIA has found that a very good symmetry grade does not impact an excellent cut grade negatively whereas an only good cut grade can.
I would therefore only recommend to choose a diamond with a “Very Good” symmetry grade. Such diamonds include symmetry flaws that can only be seen face-up at 10X magnification. Diamonds with only “Good” symmetry grades on the other hand might exhibit symmetry flaws that can be viewed with the unaided eye. Such diamonds will never receive an “Excellent” cut grade.
In my humble opinion and in the opinion of many other diamond experts you should always try to get a diamond with an “Excellent” cut grade and therefore with a “Very Good” or “Excellent” cut grade. Of course, you could also find a stone with only a “Good” symmetry grade which does not impact the light performance of a diamond in a noticeably negative way but it would be more of a gamble. The symmetry of a diamond has a direct impact on the diamond cut and the diamond cut is the only thing that you should not compromise on!
Which factors influence the symmetry grade of a diamond?
The symmetry of diamond is made up of 10 different criteria which were updated by GIA in 2012. The symmetry grade of a diamond can thus be impacted negatively by one of the following symmetry flaws:
1. Out-of-round
An out-of-round diamond deviates from the normally round diamond shape insofar as the width differs from length. A perfectly round diamond would have the exact amount of mm both in width and in length. For “Excellent” symmetry GIA allows a deviation of 0 – 0.9 % in the width-length variance.
A deviation of more than 0.9 % will be reason enough to classify the diamond into “Very Good” symmetry. An out-of-round diamond might have a negative impact on all the other facets of a diamond.
2. Table off-center
The table is considered off-center if one table edge is further away from the table center than another table edge. Gemmologist measure the direct distance between the table center and the outline center and then compare it to the average diameter.
For a diamond to fall within an „Excellent“ symmetry grade the deviation must be under 0.6 %. Otherwise the symmetry grade will be reduced to “Very Good”.
3. Culet off-center
A culet is considered to be off-center when the culet deviates from the central position of the pavilion. A diamond with a culet that is visibly of center to the naked eye should be disregarded.
If the direct distance between the culet center and the outline center projected into any horizontal plane such as the table deviates more than 0.6 % from the average diameter it will only receive a “Very Good” symmetry grade.
4. Table/culet alignment
The Table/Culet alignment symmetry flaw refers to a misplacement of the
table facet and culet in the opposite direction.
It is measured as the direct distance between the table center and the culet center projected into the table plane, as a percentage of the average diameter. If the this distance is more than 0.9 % of the average diameter the symmetry grade will be downgraded to “Very Good.
5. Crown height variation
A crown height variation refers to differing crown height measurements. In a perfect diamond the crown height should always be the same. However, due to a wavy girdle or the table and girdle not being parallel to each other crown heights can vary.
If the difference between the maximum and minimum crown height values differs more than 1.2 % from the average diameter the diamond will be downgraded to the “Very Good” symmetry grade.
6. Crown angle variation
The crown angle variation refers to the crown angles being unequal. Such a crown angle variation is usually the result of an off-center table.
The crown angle variation is simply measured as the difference between the maximum and minimum crown angle values in degrees. If the difference is more than 1.2 degrees the diamond’s symmetry grade will be downgraded to “Very Good”.
7. Pavilion depth variation
The pavilion depth variation refers to differing pavilion depth measurements. Usually such differing measurements are a strong indicator of a wavy girdle.
The pavilion depth variation is measured as the difference between the maximum and minimum pavilion depth values. If this difference surmounts 1.2 % of the average diameter the symmetry grade will be downgraded to “Very Good”.
8. Pavilion angle variation
The pavilion angle variation refers to the pavilion angles being unequal. This is usually related to the diamond’s culet being off-center.
The pavilion angle variation is measured as the difference between the maximum and minimum pavilion angle values in degrees. If the difference is more than 0.9 degrees the diamond’s symmetry grade will be downgraded.
9. Girdle thickness variation
A girdle thickness variation refers to a variation of the girdle thickness at bezel positions. The bezel position is to be found where the crown of a diamond meets the girdle.
The girdle thickness variation is measured as the difference between the minimum and maximum girdle thickness values measured at bezel positions. If the difference is more than 1.2 % of the average diameter the symmetry grade will be downgraded to “Very Good”.
10. Table size variation
The table size variation refers to differing table size measurements. This is due to the table not being octagonal.
It is measured as the difference between the minimum and maximum table size values. If this difference surmounts 1.2 % of the average diameter the stone’s symmetry grade will be downgraded.
Please read on if you want to know more about the Polish of a diamond.
