When it comes to buying diamonds, GIA and AGS are the only labs that I would consider and recommend to readers. Unlike other major labs like EGL/IGI or private labs/appraisers, GIA and AGS represent some of the strictest and most consistent grading standards in the industry. This means that you know what you are paying for and the characteristic 4Cs of a diamond had been properly determined.

Despite their authority in the world of diamond grading, GIA and AGS do have their subtle differences from each other. In a round brilliant cut, both labs use different nomenclature and slightly different methodology in determining the cut grade of the stone. At the top tiers of each lab, diamonds are graded triple Excellent by GIA and triple 0s by AGS.

 
GIA Triple Ex AGS 000
Excellent Cut Ideal Cut
Excellent Polish Ideal Polish
Excellent Symmetry Ideal Symmetry
   

Some of the most common questions that I get from readers are usually in the line of: “Should I choose an AGS 000 stone over a GIA 3 Ex diamond?”, “Since AGS is grading diamonds for light performance, does it mean every diamond scoring triple zeros will look great?”, “Does a diamond with GIA Excellents equals to another with AGS 0s?” and other related queries.

Well, the answers is no.

Due to the type of methodology the labs employ and the different criteria for determining cut grade, diamonds assigned with the same cut grade do not necessarily have the same level of light performance.

Here’s a fact. Not all GIA triple-excellent diamonds are created equal. Some are less brilliant than others while some are noticeably brighter. The same applies for diamonds graded by AGS although the degree of variation is somewhat lesser than GIA’s.

   
   

The table below shows the relationship between pavilion/crown angles and how the different labs tend to grade a diamond’s cut.

In the blue zones, those are “sweet spots” for AGS ideal 1s and 0s. In the red zones, it shows the proportions that are sweet spots for GIA Excellent cuts. As you can see, there are some overlapping areas whereby GIA and AGS have common ground.

proportions that determine a diamond's cut

Source: Accordance in Round Brilliant Diamond Cutting, Michael Cowing

Visual Differences Between AGS 000 Diamonds

From the face-up view of 5 diamonds below, you can actually see the differences between them even though they were all graded with the same cut by AGS. Do they look totally alike to each other? They don’t and you can let your eyes be the judge…

ags triple-zeros comparison

 
a) 1.20 Carat I Color VS1 Clarity
b) 1.33 Carat H Color SI1 Clarity
c) 0.92 Carat J Color VVS1 Clarity
d) 1.33 Carat J Color SI1 Clarity
e) 1.74 Carat G Color VVS2 Clarity
 

 

Visual Differences in GIA Ex-Ex-Ex Diamonds

Likewise, diamonds graded by GIA with the same cut grade also display different personalities and outlooks.

comparison of 5 gia ex ex ex diamonds

 
i) 1.20 Carat J Color VS2 Clarity
ii) 1.16 Carat H Color SI1 Clarity
iii) 1.01 Carat F Color VVS2 Clarity
iv) 1.00 Carat D Color VVS2 Clarity
v) 2.60 Carat J Color SI2 Clarity
 

Evaluating Optics With an Idealscope

The idealscope is a handy tool for evaluating a diamond’s light performance. In essence, we want to see more red which indicates light return and less white which indicate light leakage. The black colored regions represent light obstruction and is a factor for creating contrast within the stone.

The images below correspond to the respective diamonds that were shown earlier.

idealscope images for ags ideal cuts

Next, here are the respective images for the GIA graded stones…

various idealscope photographs for gia 3ex

With the use of optical performance evaluation tools, we can clearly see the discrepancies between the diamonds. Now, do you understand why shopping based on a certificate alone isn’t enough.

How the Different Labs View Cut Grading

To put a long story short without going into nitty gritty details, GIA determines the diamond’s cut grade using a combination of factors such as face-up appearance, proportions and craftsmanship elements (symmetry/polish). Using computer simulations and models that are based on human observations, the diamond’s brilliance is then evaluated in a series of steps.

On the other hand, AGS utilizes a system where individual facets of the diamond are measured in 3D instead of GIA’s 2D approach. They then subject these measurements to a ray tracing test to measure how light travels within the diamond. Using their proprietary ray-trace software, the light performance of the diamond is subsequently determined.

What the Lab Reports Don’t Show…

When it comes to critically assessing light performance in a diamond, both types of lab reports do not offer sufficient details for consumers to make an informed purchase.

Having said this, one of the most common misconception consumers have is that they can’t go wrong with an AGS triple-zero diamond since it is graded with light performance in mind. Well, the truth is far from that. Both of these AGS diamonds are triple 0s and one of them (No. 2) is actually showing a slight fish eye effect due to its shallow depth!

photographic examples of bad and good ags triple 0s and gia triple excellents

Click these links to see full details of the diamonds…

     
1) 1.01 Carat D Color VVS1 Ideal Cut Round Brilliant
2) 1.00 Carat F Color SI2 Ideal Cut Round Brilliant
3) 0.80 Carat I Color VVS2 Excellent Cut Round Brilliant
4) 0.70 Carat I Color SI1 Excellent Cut Round Brilliant

     


If you intend to buy a diamond “blind”, please think again. My point here is that you need to exercise due diligence and caution when making huge purchases. To learn more about making a great selection, I will reveal the insights and concepts with this step by step guide to choosing diamonds

Shopping a truly well-cut diamond requires good knowledge and a lot of effort in filtering out the poor ones. If you want to avoid the hassle and avoid costly mistakes, I recommend checking out Whiteflash and Brian Gavin. Their signature diamonds are pre-screened with stringent requirements and represent the best of the best in the industry.

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2 Comments

  1. Helder-
    September 8, 2015 at 2:59 pm

    AGS compared to GIA seems to be a much stricter lab because of the ray tracing tests that they perform. Personally, I like the idea of quantifying brightness and light performance. In contrast, the loose GIA diamonds seem to have a much broader range of cut quality.

  2. Paul Gian-
    September 9, 2015 at 10:01 pm

    I agree with your views. When most people compare gia versus ags diamonds, one of the most common mistake they make is to assume that they are equivalent. They aren’t.

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