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Diamond Grading Certificates

Lab reports are important but this tutorial is designed to show you that you simply cannot rely on a lab report for evaluating the cut of a diamond.

Most know that by this stage, you are more than likely to be aware of at least some of the different labs that grade diamonds. If you haven’t then no worries, all you need to know is that the most important ones are GIA and AGS and that GIA is known to be stricter in color and clarity and AGS is known to be stricter in cut.

Remember that you have the choices. You can choose which lab report accompanies your diamond and you can also choose the numbers that are on the report. With so many diamonds available, why take unnecessary risks when purchasing a diamond? The advice is to stick to GIA and then learn to evaluate cut yourself so you can choose a diamond that is guaranteed to receive the highest cut grades from both labs.

Unreliable Labs

Did you know that there is a disclaimer in some lab reports that there can be up to a 2-grade variation in clarity and color to what has been reported? The advice for you is to avoid diamonds graded by EGL all together.

But if you buy a diamond that has a reputable lab report then you’re safe right?

Even among the top labs, it is widely known that one grade either way is within acceptable gemological tolerances. It’s important for you to know that no diamond is guaranteed to receive the same grade if it’s graded again, even by the same lab. There is always going to be some degree of human error. This fact alone should be enough to make you a little hesitant to fully trust lab reports.

Unreliable Numbers

If you look at any diamond grading certificate it will show you a single number for each proportion. This number represents an average of all 8 sides of the diamond. This means that it is possible for a diamond representing a 35-degree crown angle to have at least 1 of the crown angle facets to be 36.2 degrees, while still remaining within the 1.2-degree variation limit that keeps the diamond within an excellent cut grade. Similarly a diamond representing a 41-degree pavilion angle can have at least 1 of the facets to be 41.9-degrees, and be within its 0.9-degree variation limit.

If you still don’t get it, then consider a diamond with crown angles of 35.0, 35.1, 35.0, 35.3, 36.2, 35.0, 35.1, 35.2. This diamond’s lab report will show a 35.0-degree crown angle on a GIA certificate but in actual fact is masquerading a 36.2-degree crown angle! Okay, so this is quite an extreme example but  it does a good job highlighting the risks.

The point is that you want to minimise this risk. Since we’re looking for a crown angle between 34 to 35 degrees, the best way to do this is to find a diamond with a 34.5 degrees, at least for a GIA diamond.

If that is not enough to convince you, then consider that these measurements are taken by a digital scanner that has a margin of error of +/- 0.2 degrees. This means that each of the 8 facets are measured, say 36.2 degrees, but the same scanner could measure it as 36.4 degrees or 36.0 on a different occasion. You can now see why GIA only reports to the nearest 0.5 degrees, because the variance in the measurement is 0.4 degrees.

AGS presents data that at first glance represent an accuracy to the nearest 0.1-degree. This is in some ways more misleading than GIA because this would suggest that the number represented is more accurate the machine that is doing the measurements, which is impossible! The 0.1-degree of accuracy is therefore meaningless and is simply an average number rounded to the nearest decimal place.

If you haven’t got it yet, it is that our current measuring technologies are limited at a variance of 0.4 degrees and that is the margin of error. An AGS graded diamond that states 34.3 degrees is therefore only accurate to within a half a degree and the same diamond on a GIA report would say 34.5 degrees.

It is misleading to think that an AGS diamond with a 34.3-degree crown angle is any closer to having more 34-degree crown angles than one that is stated as 34.7 degrees. Therefore, both AGS and GIA reports actually contain the same degree of accuracy in terms of the numbers on the proportions! A similar analysis can be done for linear measurements where the scanner accuracy is +/- 0.02mm.

Missing Information

A typical lab report from GIA and AGS will both state the following information relating to cut:

  • Shape and Cutting Style
  • Measurements
  • Cut Grade
  • Polish
  • Symmetry
  • Table %
  • Depth %
  • Crown Angle
  • Crown Height
  • Pavilion Angle
  • Pavilion Depth
  • Lower Girdle %
  • Star : Upper Girdle Ratio
  • Girdle Thickness
  • Culet

What the report doesn’t tell you are the following:

  • Upper Girdle %
  • Upper Girdle Angle
  • Upper Girdle Indexing
  • Star Angle
  • Lower Girdle Angle
  • Weight Ratio
  • Durability
  • Girdle Bezel Thickness
  • Girdle Bone Thickness
  • Girdle Valley Thickness
  • Degree of Twist

On top of this, the reports don’t tell you any information regarding several key aspects of light performance and diamond optics:

  • Scintillation
  • Fire
  • Transparency
  • Face up color
  • Body undertone color
  • Negative impact of fluorescence
  • Negative impact of internal graining


The point should be clear. Don’t solely rely on a lab report for its information relating to cut. A diamond grading certificate’s purpose is primarily for general pricing, marketing, and facilitating industry buyers who are comfortable buying on the certificate alone.

By sticking to the middle of the recommended specifications, you can minimize the risks of poor averaging and number rounding. However, there is no way of completely eliminating risk when buying blind without the aid of other information such as properly taken idealscope, ASET, and hearts images.


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