Recently, one of our readers noticed something weird in an AGS lab report for a princess cut diamond and asked me the following question: “I was browsing through James Allen for a 1 carat sized princess cut and shortlisted a couple of diamonds in their True Hearts line.
When I brought up the certificate to look at details, I noticed a description called Proportions Factors under the Cut Grade section. It was listed as AGS Very Good 2.
I’m not sure whether this is something new because I don’t see anything similar in other AGS platinum light performance reports. I hope you can shed light on this.”
First of all, let me show you the copy of the grading report in question. You can clearly see that the stone receives a rating of “AGS Very Good 2” for Proportions Factors. You can view the full details of the diamond with this link.
Shown below is an example of a grading report for a typical AGS000 ideal princess cut. You can view the corresponding listing for this diamond here. Interestingly, both these diamonds were graded at the same time period in Jan 2015.
The short answer is, the crown angle of the particular diamond is too high (48.5°). Even though the diamond exhibits ideal light return, polish and symmetry, AGS penalized this diamond because of its excessive crown angles. As a result, this diamond faces up smaller for its weight and is rightfully graded to be an AGS 2.
For people who are interested in more details, the long answer can get pretty technical. In essence, AGS classifies the diamond’s pavilion angles, crown angles, table size percentage, culet size, star facet length, lower girdle facet length, girdle thickness, weight ratio, durability and tilt collectively as Proportions Factors.
If you are wondering why AGS listed the Proportions Factor in the grading report for one diamond and not the other, my belief is that it is done purely for indication purposes. It is in my understanding that all diamonds that are subjected to an AGS grading process will undergo a proportions check.
Like GIA, AGS also utilizes some form of guidelines/charts when determining the cut grade of a diamond. In this particular diamond example, it is a 4 chevron princess cut and all the corresponding cutting chart guidelines can be found here.
Now, without a Sarin scan to tell me the actual angles/proportions of individual facets, I can only make an educated estimate as to how AGS derives a grade of “Very Good 2” for the diamond. This can be performed by using the following table provided by AGS.
* Assumption: the diamond has an upper pavilion ratio of 70% and lower pavilion ratio of 10%.
With that, I hope this clears up any questions you might have about AGS’s proportion factors. Basically, you don’t have to worry yourself with the technicalities of how the labs grade diamonds if you don’t want to.
For a consumer, you can simply stick with AGS triple ideal diamonds and generally be assured of a great looking stone.