Diamond Symmetry

To understand the importance of diamond symmetry it is helpful to define the term and appreciate what symmetry means to human perception.

sym·me·try: The quality of being made up of exactly similar parts facing each other or around an axis.

Humans respond positively to things that are highly symmetrical. Studies have shown that we are even attracted to mates whose faces are symmetrical. There is something about symmetry that pleases us intrinsically. Perhaps it is the predictability of symmetrical objects that makes it easier for us to understand and appreciate – our brains have less work to do in deciphering things that are symmetrical. Things that are chaotic in nature provoke stress and are generally not as aesthetically pleasing.

Research done at Stanford University has concluded that, “symmetry is a one of the most perceptually salient properties of visual images. Because of this, symmetry has been a recurring feature in art, architecture and other artifacts of human construction for centuries.”

In this article we will focus on symmetry as it relates to the cut quality of a polished diamond. Because a diamond is essentially a tiny sculpture made of mirrors, and those mirrors are the engine of the diamond’s light performance, the alignment and symmetry of those facets is critical to diamond beauty. In this article we will discuss the following topics:

Diamond symmetry, how it is graded in a gemological laboratory, and its impact on beauty is not generally understood by consumers. In this article we will provide a detailed overview on the topic along with advice on how to use this information in shopping for diamonds of high quality, outstanding light performance, and solid value.

Symmetry is a quality factor that is evaluated in the diamond reports issued by almost all reputable gemological laboratories. This aspect of diamond cut quality reflects the skill and care of the cutter in placing all the facets on the diamond in proper alignment with one another- known more specifically as “meet point” symmetry. A diamond grader determines if the contiguous facets all meet one another at the correct points around the diamond. Symmetry is a measure of craftsmanship and also impacts the light performance and eye-appeal of the diamond.

The term symmetry is also commonly used in the evaluation of a diamond’s optical precision. This aspect of symmetry pertains to the alignment of corresponding facets in 3D space. This is a level of symmetry beyond the scope of most laboratory reports and pertains mainly to round brilliant cut diamonds. The term Hearts and Arrows is used to describe those diamonds that have exceptional optical precision or “optical symmetry”, which can be assessed by certain types of advanced imaging such as IdealScope, Hearts and Arrows scope, and ASET. This is also referred to by some as 3D symmetry.

This type of symmetry has a significant effect on actual light performance. For a diamond to have optimal light handling properties the corresponding facets must be in precise alignment for them to act in harmony as a system of tiny mirrors designed to reflect and refract light in accordance to the facet design. Below are light performance images of two GIA Triple Ex diamonds, both having received Excellent symmetry grades but only one having top optical symmetry.

As mentioned above, a symmetry grade is rendered on almost all laboratory reports. GIA grading uses a scale that has Excellent as its top grade, then Very Good, Good, Fair and Poor. Excellent symmetry is now the default expectation of the market for round brilliant diamonds. The GIA Triple Excellent – Excellent Polish, Symmetry and overall Cut Grade, has become a baseline standard for a well-cut round diamond since the GIA cut grade system introduced their in 2005. Some meet-point symmetry faults that would drop a diamond to Very Good may be minor enough not to affect the beauty in an appreciable way, but still indicate less than optimal craftsmanship.

Besides the meet points of contiguous facets, there are several other potential symmetry faults. Off-center culet, wavy girdles, misshapen facets, uneven crown facets, and twist between the crown and pavilion are some of the other issues that can result in deductions. These issues can be seen by the grader when significant, but are largely caught in the initial scanning stage of the grading process. A highly accurate non-contact scanning device such as a Sarine machine is used to get a broad range of measurements and perform calculations to determine proportions and percentages. Metrics that are out of pre-determined tolerance limits result in symmetry grade deductions. Below is a description of the ten measurable symmetry features that GIA calculates from the scan, along with a chart of their individual tolerance limits.

Because symmetry faults result in variations in measurements, creating a system of grading based upon quantified limits to those variations enables the grading of symmetry to be largely done by machine. Whereas in earlier times symmetry was judged entirely by the human grader, beginning in 2012 symmetry features can be gleaned from the data acquired from the scan at the beginning of the grading process at GIA. The result is a more accurate and consistent method of grading diamond symmetry. By establishing benchmarks and limits on symmetry features, GIA is able to provide diamond manufacturers with detailed guidance on achieving the top symmetry grade. This has helped improve cutting in general as those manufacturers who aim for GIA Triple Ex have the tools to consistently earn the top grades.

Assigning the final symmetry grade still involves a visual assessment by the grader. While it is possible for none of the measured symmetry features to be severe enough to result in a symmetry grade deduction, small faults may combine to impact visual appearance in a way that would require a downgrade. Likewise, some features can compensate for one another visually, but if any one feature is over the limit established for machine grading of symmetry, the diamond cannot receive the top grade of Excellent. In this way the measurement system both supports and constrains the human grader, promoting consistency and accuracy.

As mentioned above, the GIA triple ex is now the modern baseline for a well-cut stone. While the GIA Excellent overall cut grade is very broad and forgiving and includes a wide range of proportions, polish and symmetry grading is somewhat more restrictive. Any symmetry faults noticeable to a grader working under 10x magnification will likely result in a downgrade from Excellent. So, even a Very Good in symmetry will keep the diamond from being a Triple Ex. And since that is now an expectation for a well-cut stone, many shoppers will pass on it, thereby reducing demand and market value.

Shoppers looking for best price may find some ‘deals’ on diamonds with less than excellent symmetry, and determine that the symmetry deduction does not impact visual beauty. In that case it may make sense to compromise a little on this aspect. However, if liquidity (the ability to sell or trade the stone in the future) is at all part of the calculus it is wise to stay with Triple Ex. Again, this has become a standard for well cut rounds. The market expects it and penalizes those stones without this pedigree. It should be noted that fancy shapes are not treated in as rigid a way in this respect.

Since many fancy shapes have multiple facet arrangements and modifications, symmetry grading is not quite as restrictive. However, balance between the quadrants of the diamond is still scrutinized closely. The culet or keel must be well centered or the diamond will look crooked. This will result in a deduction of the symmetry grade.

It is not uncommon for fancy shape diamonds to have Very Good or even Good symmetry grades. Such deductions are not as significant to the overall value of a fancy cut. However, a symmetry downgrade on a grading report should be a clue to inspect the diamond carefully to ensure that eye appeal is not diminished. A diamond with symmetry faults may always look crooked in a setting, even if it has been set correctly. Light performance can also be impacted and should be reviewed by an expert.

Below is an example of an emerald cut with outstanding optical symmetry and overall light performance.

As we have seen above, judging a diamond’s symmetry involves carefully assessing a number of factors. Many of these factors can be difficult to assess individually by the naked eye. In most cases, 10x magnification is necessary, and generally speaking a trained eye is required even when magnified. Because symmetry factors can be quantified by accurate measurement of the diamond’s parts, a high quality 3D scan of the diamond is used to pick up symmetry faults. This is why a diamond report from a top tier gemological laboratory is critical to evaluating symmetry, and the purchase of any significant diamond should be accompanied by one.

Only when symmetry is poor is it easily recognizable with the naked eye, even to casual observers. A wavy girdle is perhaps the most common symmetry fault that consumers can recognize consistently.

Another way to be able to see the quality of symmetry of a diamond is in the precision visible in advanced light performance images such as IdealScope, Hearts and Arrows scope, and ASET. Though you cannot usually pinpoint a grade for symmetry looking at these images, they will show you how much or little precision is evident in the cut quality. These images are also valuable for displaying other faults that may have a bearing on the beauty of the diamond including brightness, contrast and light leakage.

Diamond symmetry is an important aspect of cut quality and craftsmanship. The degree of symmetry in the cut impacts both its grade on a laboratory report and it’s value in the market. It also has a bearing on light performance and beauty as the precision of the cut determines whether the diamond is optimized for fire and brilliance. Because symmetry faults are difficult for consumers to assess on their own, a laboratory report which accurately measures symmetry features and is the basis for the symmetry grade, is indispensable to the successful purchase of any significant diamond.

Diamond symmetry is evaluated on a number of different factors. Facets should meet at the proper places all around the stone. This ‘meet point’ symmetry is what is graded on a diamond report. It also involves determination of such things as a centered culet, equal crown height, and a girdle that is level and not wavy. Another type of symmetry that is often discussed as “optical symmetry”, which is the degree of facet alignment in three dimensions. This type of symmetry, while very important, is not generally graded on most lab reports, and is better referred to as “optical precision” to avoid confusion. Hearts and Arrows diamonds are those with the highest degree of optical precision and can be visually evaluated using imaging such as IdealScope, Hearts and Arrows viewer and ASET.

The default standard today for a well-cut diamond is the GIA Triple Excellent, which has the top grades from GIA on polish, symmetry and overall cut grade. When shopping for natural round brilliant diamonds, a GIA Triple Ex report is a smart baseline pedigree. Rounds falling short of this standard should be expected to trade at lower prices and have limited liquidity and stored value.

Fancy shape diamonds have many facet arrangements and modifications and are not valued as strictly in the market based on their symmetry grade. But broad measures of symmetry are still important as significant symmetry faults can make the diamond look lopsided or crooked in a mounting, even when properly set.