How to Judge Diamond Light Performance

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May 2

Diamond light performance reflects cut quality, including proportions, polish, symmetry, and optical precision. The sum effect of these characteristics determines the amount of light leakage and intensity of the sparkle factor.

It's essential to recognize that the diamonds' proportions dictate the volume of light return and balance of brilliance and dispersion. Simultaneously, the optical precision or lack thereof affects light return and the intensity of the sparkle factor.

People frequently confuse optical precision with the meet-point Symmetry grade on the lab report. Diamond light performance depends on proportions and optical precision, which is the consistency of facet shape, size, and 360-degree alignment.

Black by Brian Gavin Diamond Visual Performance.

Brian Gavin Black Light Performance.

Consequently, the diamond grading laboratories do not account for optical precision in the overall cut grade. However, you can use ASET, Hearts and Arrows Scope, and Ideal Scope images to judge optical precision.

How to Judge Diamond Light Performance Online:

"Can you explain how to judge diamond visual performance online? I'm having difficulty deciding between a few since I can't view the stones for performance. Thanks in advance and for all the useful advice you provide." — Dave.

The AGSL is the only laboratory that grades diamond light performance using their Angular Spectrum Evaluation Tool (ASET). In contrast, the GIA's overall cut grade does not account for diamond light performance.

As that may be, the ASET map on an AGS Diamond Quality Document (DQD) is insufficient because it's a rendering. Specifically, the ASET map is a compilation of data derived from scans taken in multiple positions.

The supercomputer at the AGSL compiles the data and spits out a computer-generated rendering. That's all well and good, but it doesn't enable us to accurately determine the amount of light leakage.

ASET Photographs vs. Printed Maps:

Once again, the ASET map on a DQD is a rendering printed on a solid sheet of paper. In that case, it's impossible to judge diamond light leakage accurately because it's a solid object.

In contrast, an ASET photograph shows differences in hue, saturation, and intensity of color. Under those circumstances, light leakage is evident, and we can also see whether light is reflecting evenly.

Consequently, this is the moment where some people point out the obvious, which is that the AGS grades light performance. In that case, the AGS Ideal-0 overall cut rating should be sufficient.

However, each grade encompasses a range or spectrum of possibilities. There is a low, middle, and high range of performance in that case. The question then becomes, what level of diamond light performance do you seek?

Diamond Light Performance Characteristics:

Black by Brian Gavin Diamond Light Performance.

Black by Brian Gavin Light Performance.

The first thing to remember is that your diamond's sparkle factor and light performance depend on the cut quality. That might seem obvious, but there is more to the picture than meets the eye.

Spectacular light performance and sparkle factor require specific proportions and precise optical precision. If you want the very best, adhere to the ratios we recommend and hearts and arrows diamonds.

The slightest variation in proportions or optical precision produces light leakage and reduces the intensity of the sparkle. When people make the critical mistake of only relying on the lab report, they get burned.

"But The Cut Grade Is GIA Excellent"

Once again, the information on the lab reports or diamond grading "certificates" is insufficient. For example, they do not account for optical precision or azimuth shift. Consequently, the overall cut grade on the lab report only takes the following factors into account:

Major gemological laboratories like the AGS and GIA do not evaluate Optical Precision when determining the cut grade of a diamond. That seems counterintuitive since it has a significant impact on visual performance.

Fine-tune Your Search for Diamond Light Performance:

It helps to know what you're looking for if you're searching for diamonds online. Otherwise, you could waste time sifting through thousands of options that exhibit subpar performance.

Surprisingly, most AGS Ideal and GIA Excellent cut diamonds leak more light than we deem acceptable. The culprit is usually a combination of proportions and variations in facet size, shape, or alignment.

You might notice that we keep circling the importance of optical precision if you're paying attention. Believe it or not, we're not entirely daft, but we have reservations about some diamond cutters.

Why? Because they either keep missing the mark or aim for the lowest rung on the ideal target for diamond light performance. Essentially, they're sacrificing the sparkle factor to retain carat weight and yield higher profits.

Best Proportions for Round Ideal Cut Diamonds:

Remember that the AGS Ideal and GIA Excellent cut ratings encompass a broad range of potential. The middle of the spectrum is the target range we're aiming for in terms of proportions.

  • Total depth between 59 – 61.8%.
  • Table diameter between 53 – 58%.
  • Crown angle between 34.3 – 35 degrees.
  • Pavilion angle between 40.6 – 40.9 degrees.
  • Girdle between 0.7% thin to slightly thick.
  • Culet: GIA "none” or AGS "pointed” (same thing).
  • Polish of AGS Ideal or GIA Excellent.
  • Symmetry of AGS Ideal or GIA Excellent.

The proportions above set the stage for a high volume of light return. It should also produce a virtual balance of brilliance and dispersion. Any variation from the proportions above may significantly reduce the diamond's visual performance.

Be that as it may, the degree of optical precision will also influence the sparkle factor. In other words, the diamond may still leak light if the facet structure is inconsistent.

Perspective Is the Pinnacle of Understanding:

Momentarily, we'll look at the four diamonds that David referenced explicitly in his email. We'll also walk through the selection, or rather the elimination process.

First, we'll look at the diamond's proportions, polish, and symmetry grades. We'll also get into the nitty-gritty hidden factors of evaluating diamond visual performance.

Therefore, we'll look at the hearts and arrows pattern images. We will also evaluate the ASET and Ideal Scope images. Keep in mind that all four diamonds have an overall cut grade of AGS Ideal or GIA Excellent.

Under those circumstances, they are among the Top 1% of the annual production for round brilliant cut diamonds. So, they're all excellent options in their own right. However, we're looking for "the one" that is the Cat's Meow.

Option #1 - Diamond Light Performance Evaluation:

Using Ideal Scope to Judge Diamond Light Performance.

Ideal Scope for James Allen GIA Excellent Cut.

This 1.62 carat, H-color, VS-2 clarity, GIA Excellent cut round diamond from James Allen is the first option. We wouldn't consider this diamond because the total depth exceeds our selection criteria.

We would not discuss this diamond if the client did not ask about it specifically. The 62.3% total depth hides valuable carat weight in depth rather than being visible as outside diameter.

In addition, the 41-degree pavilion angle will produce less light return than 40.6 to 40.9 degrees. Consequently, the light pink and semi-translucent areas visible under the table facet indicate light leakage.

Under those circumstances, we know the 41-degree pavilion angle is not optimum. But, we also know that there are optical precision issues.

In that case, the 41-degree pavilion angle is a factor, but so is the degree of optical precision, as will be shown below.

Use this link to Search James Allen using our presets for total depth and table diameter. Then, you only need to verify that the crown/pavilion offset is in the range we recommend above.

Use an H&A Scope to Judge Optical Precision:

Evaluating Diamond Visual Performance Using Heart Pattern.

Standard Ideal Cut Diamond from James Allen.

The Hearts image shown here indicates a variance in the size and shape of the hearts. Simultaneously, the hearts' tips twist and bleed into the arrowhead-like shapes in the middle.

That indicates a difference in the length of the lower girdle facets. There are also indications of Azimuth shift, a variance in the facets' alignment.

Consequently, it requires a lot of skill to align the diamond facets properly from the perspective of 360-degrees. It can also take up to 4X longer to polish a diamond to exhibit a higher degree of optical precision.

For that reason, James Allen True Hearts Diamonds cost more than standard ideal alternatives. On a positive note, better optical precision produces more virtual facets and more intense sparkle.

Through the Kaleidoscope of Virtual Facets:

How Virtual Facets Contribute to Diamond Visual Performance.

Virtual Facets the Building Blocks of Sparkle.

Virtual facets are internal reflections of light similar to what you might see when looking through a kaleidoscope. Only in this case, the various shapes result from light reflecting through the overlapping facet pattern.

Under those circumstances, more precise hearts and arrows patterns produce better diamond light performance. The resulting sparkle factor will also be more vivid and intense.

Consequently, we would reject this diamond because the inconsistency in the hearts' pattern contributes to light leakage. From our perspective, there is no need to settle when better options are available.

Finally, the diamond's primary inclusions consist of clouds, crystals, feathers, and needles, which are acceptable. We recommend avoiding bruises, cavities, chips, etch channels, knots, and laser drill holes.

Judging Diamond Light Performance - Option #2:

Diamond Light Performance Ideal Scope.

Ideal Scope GIA Excellent cut diamond.

The second diamond on the list is a 1.66 carat, I color, VS-2 clarity round ideal cut diamond from James Allen. This diamond would typically not cross our path because the total depth is 62.1%.

Nevertheless, this diamond is more interesting because the AGS Light Performance grading platform is more insightful than what the GIA provides.

In this case, the 35-degree crown angle is a reasonable offset for the 40.9-degree pavilion angle. Consequently, that sets the stage for a high volume of light return and a virtual balance of brilliance and dispersion.

The Ideal Scope image reveals moderate light leakage, as evident by the light pink and semi-translucent sections. This amount of leakage is typical of standard ideal cut diamonds that lack higher degrees of optical precision.

Inconsistent Hearts' Pattern:

Irregular Hearts due to variations in Optical Precision.

James Allen GIA Excellent Cut (not H&A)

There is a fair amount of inconsistency in the hearts' pattern despite the proportions. Specifically, you'll see that the hearts' are of different sizes and shapes.

Plus, the space around each heart is inconsistent, creating some of the leakage shown above. If you look closely, you will also see that the hearts' tips appear to be twisting.

Read our tutorial on hearts and arrows diamonds for an in-depth explanation and illustration. The short answer is that it indicates a difference in the length of the lower girdle facets.

The plotting diagram on the lab report indicates the presence of crystals. That means that the diamond crystal embodied other smaller diamonds as it formed. In that case, the inclusions are of no consequence, and I'm inclined to keep this diamond on the list.

Option #3 - Review of Diamond Visual Performance:

Diamond Visual Performance Ideal Scope Very Good to Excellent.

Very Good Ideal Scope James Allen GIA 3X.

The third option is this 1.68 carat, I-color, VS-2 clarity, round ideal cut diamond from James Allen. It has an overall grade of AGS Ideal-0 on the AGSL's Light Performance grading platform.

The total depth of 62.2% is grounds for automatic rejection based on our selection criteria. However, the 34.9-degree crown angle is a desirable offset for the 40.8-degree pavilion angle.

In that case, the diamond might exhibit a high volume of light return and a virtual balance of brilliance and dispersion. The Ideal Scope image certainly looks better than the other two options above.

If you look at this Ideal Scope reference chart, I think you'll agree that it falls between good and excellent. There is room for improvement, and I suggest we keep looking.

The Effect of Optical Precision on Light Performance:

Evaluating Diamond Visual Performance Using H&A Scope.

James Allen GIA Excellent Cut H&A Scope.

As you can see, there is more consistency in the size and shape of the hearts. Only a little twisting is visible and minimal differences in size and shape.

However, the hearts' pattern still lacks the consistency that produces exceptional light performance. That explains the inconsistency of hue and saturation visible under the table facet in the Ideal Scope image.

Hopefully, you are beginning to see the effect of optical precision on diamond visual performance. Significant variations in the size, shape, and spacing of the hearts will create light leakage.

For that reason, I think you should limit your search to hearts and arrows super ideal cut diamonds. Assuming that diamond light performance is an essential factor.

Advanced ASET Provides Additional Insight:

Brian Gavin Signature Diamond Visual Performance ASET Map.

Advanced ASET for Brian Gavin Diamond.

This 1.726 carat, I-color, VS-1 clarity, Brian Gavin Signature diamond is the fourth option. The proprietary light performance grading report from the AGS Laboratory features the Advanced ASET 30/40 map.

The additional data that Advanced ASET provides makes it easier to judge diamond light performance. However, I still prefer an actual photograph since the lab report's ASET map is computer-generated.

In that case, you can't use the ASET map to determine the extent of light leakage. Fortunately, Brian Gavin provides photographs of the diamond as seen through an ASET and Hearts & Arrows Scope.

When we combine those images with the high-resolution video's insight, it's evident that this diamond is the best choice.

How to Judge Diamond Visual Performance Using ASET

ASET for Brian Gavin Signature Diamond.

High Resolution Video for Brian Gavin Signature Diamond.

Ultra-high Resolution Video for Brian Gavin Diamond.

Diamond Visual Performance is a Factor of Optical Precision.

Optical Precision of Brian Gavin Signature Diamond.

About the Author

Todd Gray is the CEO of Gray Matter Development, LLC., DBA Nice Ice Diamonds. He is also a trade consultant and diamond buyer with 35+ years of experience specializing in light performance. Todd also ghostwrites educational content for online ventures and is an avid Freediver.