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.
If the same diamond with 3 excellent grades in GIA is sent to AGS, it might not necessarily be graded as a AGS triple-0 stone.
Vice versa, a 000 AGS diamond might not necessarily attain a triple excellent GIA grade.
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.
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…
Likewise, diamonds graded by GIA with the same cut grade also display different personalities and outlooks.
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.
Next, here are the respective images for the GIA graded stones…
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.
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.
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!
Click these links to see full details of the diamonds…