Interpreting AGS cert

[dmus]

I have an AGS cert. When I add the crown %, pavillion % and girdle % (a range) they don''t add up to the depth %. What don''t I understand? I beieve I don''t understand the girdle % but don''t know what it is then.

 

[WinkHPD]

The girdle percentage is a range from the thinnest to the thickest. plus the diameter will normally have some variance and the % are based on the average diameter.

Wink

 

[oldminer]

If they really gave you all the numbers that could be measured, there would be so many variables that it would fail to even be a decent screening tool for cut quality range. By giving the important variables you have enough to do the rough screening that parameters are good for while limiting the potential for worthless confusion.

Of course, not giving an explanation of how the numbers are dummied down makes for its own version of confusion. I have found that the simple sets of numbers usually found now in quality lab reports very useful to the explanation of what probably is going on with the appearance and cut quality of a diamond. To give many more numbers would not provide better screening or better use of these reports.

I think the critical numbers are:
min-max diameter so that you can see if there is diameter variation.
crown height, crown angle
table percentage
girdle min and max thickness (words are sufficient)
pavilion depth, pavilion angle
culet size (in words is sufficient)

Additional important facts:
polish and symmetry grades
bruted, polished or faceted girdle comments
surface graining, twinning, colored internal graining, indents, naturals, extra facets
strength and color of UV fluorescence

Beyond these you must have carat, color, clarity, and outline shape to determine the value.
The weight and color with AGS are going to be pretty much dependable. Clarity grading is also very good, but there are high, middle and low of any clarity types below the VVS stones. Black spots have more impact than white spots. Open blemishes have more impact than totally internal ones. Outline shape is not only the basic "round", "pear", "princess" etc, but the beauty and symmetry of the outline for many shapes. You want not only a pretty, sparkly diamond, but one that has a flowing look with beauty in its basic outline, too.

There''s a lot to the description of diamonds. The numbers have importance, but they are often trumped by the weight of the many other details that must be examined. It is a balanced approach which works best.

 

[dmus]

Thanks. But if the crown and pavillian per cents are based on the average diameter with the girdle % they should all add up to the depth %. But it''s not even close. I get 58.4% and the depth per cent is 60.1%. Are the percentages just that in accurate. And I''m using the max girdle % and still coming up short by what I consider a large %.

My wife would say I''m too annal. But when things don''t add up I just want to understand why. Thanks again.

 

[oldminer]

You are the "engineer" and she is the one going to wear it. You can overanalyze, but the diamond is more like a piece of art hanging on the wall of a museum. It has romantic meaning and beauty which is not defined by how it measures. If you want VERY accurate measures, a better machine such as Helium or GL3100 by ImaGem can provide very tight data and high details. THese devices are made fby engineers who assess measures and attach beauty as a by-product.

There is nothing inherently wrong with people who seek numeric perfection, but it is wel beyond the scope of average consumers. People can re-measure such a diamond for you and give ou truer data should you wish to pursue it.

 

[denverappraiser]

Date: 1/18/2008 1:04:19 AM
Author: dmus
Thanks. But if the crown and pavillian per cents are based on the average diameter with the girdle % they should all add up to the depth %. But it''s not even close. I get 58.4% and the depth per cent is 60.1%. Are the percentages just that in accurate. And I''m using the max girdle % and still coming up short by what I consider a large %.

My wife would say I''m too annal. But when things don''t add up I just want to understand why. Thanks again.

A girdle profile is a scalloped edge that consists of peaks and valleys. Within this, some peaks are higher than others and some valleys are lower than others resulting in thick and thin spots. Extra facets, naturals and other surface features can modify this as well. Different labs do this differently but the two girdle measurements you see on an AGS report are the maximum and minimum valley and reported as a percentage of the total depth and rounded to the nearest 1/10%.

The crown height for each crown main facet is the measurement from the point of the facet where it meets the girdle to the table plane. It’s not the length of the facet but the other leg of the triangle, the height. These facets touch the girdle at a peak. There are 8 of these. The number you see reported is the average of these 8 figures rounded to 1%.

The pavilion depth is the same general approach as the crown height. It’s the average of the depths of the 8 pavilion mains as measured from the girdle to the culet. They too meet the girdle at the peaks.

There are 16 peaks and related valleys on a standard round brilliant cut. We’ve just discussed 8 of them. The other 8 come from the upper and lower girdle facets and these may end up being what actually drives the max and min as well as the average girdle number. Depending on how these are done they can result in a minimum that’s smaller and an average that’s either smaller or larger. This information is not included on a standard AGS report (or any other major lab document).

What you’re observing comes from four sources. The first is the difference between the height of the peaks and valleys on the girdle. The second is the inherent problems that come from all of this averaging and rounding. Third is that these can be amplified by symmetry variations in the stone in cases where the table and girdle aren’t exactly parallel or when the culet isn’t in the center. Fourth is simply calibration of the equipment and the accuracy of the individual measurements themselves.

Neil Beaty
GG(GIA) ICGA(AGS) NAJA
Professional Appraisals in Denver