The symmetry grade of a diamond refers to the external balance and alignment of the facets. When you look at real diamonds, girdles are not always perfectly round and tables are not always perfect octagons. Sometimes facets have shapes that are distorted and sometimes they appear to have lopped off points. Infrequently, the table tilts towards the girdle plane; sometimes it is off-center, and sometimes the girdle looks more like a warped record than a flat disc. The culet is not always properly centered and on occasion, the bezels and pavilion mains may not be directly opposite each other. These are aspects of symmetry.
Here are some of the proportion-related symmetry features.
- Out-of-round: Often due to flattened area due to a natural or extra facet
- Off-center table: Caused by bezel facets on crown being different sizes
- Off-center culet: Corresponding pavilion facets are not the same size
- Table/culet Misalignment: Displacement of the table and culet in opposite directions
- Table and girdle not parallel
- Wavy girdle
- Girdle thickness variation
- Crown angle variation
- Pavilion angle variation
Here are some of the facet-related symmetry features.
- Misalignment of the crown and pavilion facets
- Non-pointing: Facet points to not reach prescribed location or are not completely finished
- o a point
- Misshapen facet: Different shapes or sizes for facets that are supposed to match
- Non-octagonal table: Misshapen star and bezel facets around the table cause an irregular shaped table
- Missing facet: Facet is missing that would make pattern asymmetrical
- Natural: Part of the original rough diamond’s surface that remains on the polished diamond (usually on the girdle)
- Extra facet: Additional facet placed without regard to pattern symmetry or standard cutting style
The GIA symmetry grade
- Excellent: Ranges from no symmetry features to microscopic features that are difficult to see at 10X magnification.
- Very Good: Minor symmetry features are seen face-up at 10X magnification.
- Good: Noticeable symmetry features are seen face-up at 10X magnification and the diamond’s overall appearance might be affected with the unaided eye.
- Fair: Obvious symmetry features are seen face-up at 10X magnification and the diamond’s overall appearance is often affected with the unaided eye.
- Poor: Prominent symmetry features are seen face-up at 10X magnification and the diamond’s overall appearance is significantly affected when viewed with the unaided eye.
Diamonds with excellent symmetry may have one or two tiny, inconspicuous symmetry discrepancies. If a stone has a few small or slight symmetry problems, its rating is very good. Symmetry issues that are noticeable under 10X magnification call for a rating of good. If misalignments are easy to see, the grading is fair. If there are any major symmetry variations, the rating is fair or poor.
AGS uses a scale of 0 to 10 to rank symmetry as follows:
- 0 – Extremely difficult to locate under 10X magnification
- 1 – Very difficult to locate under 10X
- 2 – Difficult to locate under 10X
- 3 – Relatively easy to see under 10X (not visible to the unaided eye)
- 4 – Easy to see under 10X (extremely difficult to see with the unaided eye)
- 5 – Very easy to see under 10X (very difficult to see with the unaided eye)
- 6 – Obvious under 10X (difficult with the unaided eye)
- 7 – Relatively easy to see with the unaided eye
- 8 – Easy to see with the unaided eye
- 9-10 – Obvious to see with the unaided eye
Just as most polished diamonds have some blemishes, most have at least a few symmetry discrepancies, too. Like blemishes, most symmetry discrepancies have little effect on appearance, at least to the unaided eye.
There is another type of symmetry related to diamonds. This is the Optical Symmetry that is displayed when viewing a diamond with a Hearts & Arrows viewer, Ideal-Scope or ASET scope. Optical Symmetry is the visual result of how the facets of a diamond align with each other internally and how light reflects within the diamond.
Do not confuse the Symmetry grade on a laboratory report with the Optical Symmetry seen with a Hearts & Arrows viewer. Many diamonds with Excellent symmetry grade do not display a Hearts & Arrows pattern and there can be beautiful Hearts & Arrows pattern with diamonds that do not have Excellent symmetry grade.