How wide should a girdle be on a round brilliant diamond?
“Please tell me how wide the girdle should be on a round brilliant diamond. I’m considering buying a diamond from a local jewelry store. It weighs 2.08 carats, is I-color and VS-2 clarity, GIA Excellent cut. The measurements are 8.02 x 8.08 x 5.11 mm, with a total depth of 63.4% and a 57% table diameter. The pavilion angle is 41.3 degrees and the crown angle is 35.5 degrees. It has a thick girdle and no culet. The diamond is GIA Excellent cut, which I assumed would mean that this is a really good diamond. However, when I punch the numbers into the Holloway Cut Adviser, the diamond gets a score of 4.9 good, and it only scores fair for fire and scintillation? Can you shed some light on this?”
HCA does not take girdle thickness into account:
The first thing to realize is that the Holloway Cut Adviser does not take girdle thickness into account. The score of 4.9 Good that it estimated for this diamond is based upon the total depth, table diameter, crown angle and pavilion angle measurements.
The total depth of this diamond is pretty steep by our standards. The extra total depth is the result of hiding carat weight within the pavilion (lower half) and girdle sections of the diamond. The result of this is that the visible outside diameter of the diamond is going to appear small for the carat weight, because the cutter is hiding carat weight in the depth.
How girdle thickness and depth affect diameter:
Take a look at the outside diameter for this diamond. According to the GIA, the diamond measures 8.02 x 8.08 x 5.11 mm. Add the first two measurements together and divide by two to get the average outside diameter, this will be how large the diamond faces-up.
8.02 + 8.08 = 16.1 / 2 = 8.05 mm average outside diameter, the visual portion of the diamond. Keep in mind that this diamond weighs 2.08 carats.
Now take a look at the measurements for this 2.00 carat, H-color, VS-2 clarity, Brian Gavin Signature round diamond. According to the AGS Laboratory, this diamond measures 8.06 X 8.11 X 5.01 mm. So again, we’re going to add the first two measurements together and divide that sum by two: 8.06 + 8.11 = 16.17 / 2 = 8.085 mm.
Why does the 2.00 carat diamond face-up larger than the 2.08 carat diamond?
Well for one thing, because the diamond cutter cut this diamond with a thick girdle edge. Based upon the criteria for a thick girdle on the GIA grading system, the valleys of the girdle edge are likely to be around 3.5% and the peaks will be around 5.2% which amounts to a lot of carat weight.
Keep in mind that on the AGS grading scale, a medium girdle is going to measure about 3.3% and a slightly thick girdle will go up to about 4.5% which will have very little effect upon the total depth or diameter. Of course, things are not quite so simple since we’re talking about two different grading scales.
How AGS & GIA measure girdle thickness:
The American Gem Society (AGS) and Gemological Institute of America (GIA) measure and report diamond girdle thickness differently. The AGS measurement is based upon the thinnest and thickest places that occur at any point of the girdle edge. This means that the AGS is measuring both the valleys (low sections) and the peaks (high points) of the girdle edge.
While the GIA is only reporting the thinnest and thickest measurements of the valleys (low sections) and that means that the girdle edge is probably thicker than you might imagine.
If you think of the girdle edge of a diamond as symmetrical ocean waves, the outline of which is mirrored on two sides, which represent the upper and lower portions of the girdle. This means that the GIA is only measuring the depth of the girdle edge at the low point, but not where it crests to full height.
Diamond cutters who focus on retention of carat weight love this, but the thing to realize is that this practice benefits them at your expense. You end up paying a premium for diamond carat weight that is hidden within the depth of the diamond.
Paying a premium for hidden carat weight:
Remember a moment ago, when I demonstrated that this 2.08 carat diamond faces-up smaller than the 2.00 carat diamond that is cut to better proportions. The 2.00 carat diamond has an average outside diameter of 8.085 mm, while the 2.08 carat diamond has an average outside diameter of 8.05 mm.
Now, I don’t know how much the 2.08 carat diamond which is deep costs, but let’s use the price of the 2.00 carat, H-color, VS-2 clarity, Brian Gavin Signature round diamond to demonstrate how much hidden carat weight costs you.
The diamond is priced at $26,417.00 for payment via wire transfer, that’s the base price of the diamond. We’re going to divide that amount by the carat weight to determine the price per carat (PPC).
26,417.00 / 2.00 carats = 13,208.50 per carat. Now imagine that this diamond actually weighed 2.08 carats, you’d multiple the PPC x 2.08 carats to get the new wire price of $27,473.68 which is a difference of $1,056.68
But in this particular instance, you’re buying a 2.08 carat diamond that faces-up the same as a 2.00 carat diamond with better proportions. So you’re not actually buying a larger diamond, you’re simply paying a premium for a diamond that looks smaller than it would if it were cut better. And that extra $1,056.68 slides right out of your bank account and into the cutters’ pocket.
