What Is Average Girdle Diameter? Why Is It So Important?

Since the invention in the 1950s by GIA, the 4Cs grading system has become the internationally recognized standard for evaluating diamonds. While all 4Cs are significant and every single "C" interacts with each other to impact the overall diamond quality, cut is the most critical factor, playing a vital role in a diamond's brilliance, fire, and scintillation.

Whether you are making your first diamond purchase or picking another sparkler for your partner, you must keep in mind that how well a diamond is cut has a close bond with the diamond's proportions. When you begin exploring, we suggest first familiarizing yourself with the terminology "average girdle diameter." What is it? Why does it matter in diamond grading?🤔 Let's find out!

Basics: What is the average girdle diameter?

Before presenting a precise definition, we need to recap the diamond girdle. When looking at a diamond from above, the girdle creates its silhouette. From a side-view, it is the thin perimeter of a diamond, dividing the crown from the pavilion below.

In round diamonds, there's not a perfectly round shape. Thus, each diameter is not the same, and we should get the average value.

However, the average girdle diameter does not calculate the average value of all the possible girdle diameter measurements. Instead, it only picks the minimum and maximum values and then proceeds to the average calculation.⚠️

How do you measure the average girdle diameter?

Gemologists use a millimeter gauge to calculate the girdle diameter of a round brilliant cut diamond. Generally, they start from an edge point where the lower tip of a bezel facet and the girdle meet to the opposite edge point, traveling through the center. Alternatively, they can start from an edge point where two upper half facets meet—cross the center—to the diagonal edge point.

Animation: How to Measure Girdle Diameter

In this manner, gemologists can obtain approximately 4-8 diameter measurements. Employing the formula below, they ultimately gain the average girdle diameter. But the value needs to be reported to the nearest thousandth of a millimeter (0.001mm) for other following computations.

Average Girdle Diameter = (minimum diameter + maximum diameter) / 2

How does average girdle diameter connect to other proportions?

Why do we say that average girdle diameter is so important? The quick answer is that it constructs the baseline of a diamond's anatomy to compute many different proportions of the diamond in the modern diamond industry. A quick skim through the table below shows that average girdle diameter is an indispensable element in various proportion calculations.

ProportionFormula
Total Depth Percentage(depth / average girdle diameter) x 100
Table Percentage(average table size / average girdle diameter) x 100
Crown Height Percentage(average crown height / average girdle diameter) x 100
Pavilion Depth Percentage(average pavilion depth / average girdle diameter) x 100
NOTE
Several centuries ago, when antique cut diamonds (e.g., rose cut, old-mine cut, Old-European cut, etc.) were dominant, girdle diameter did not have such importance to proportions. The fundamental cause is that nearly all the crafting processes of diamonds were manipulated by labor. Apparently, during these periods, "cutting accuracy" could not compete with the level achieved by advanced technologies today. Consequently, it limited in-depth research on ideal proportions within diamonds.

Is "average girdle diameter" used for fancy cut diamonds?

Unless you’ve forgotten your math lessons, you will definitely understand that there's no such terminology for fancy cut diamonds because "diameter" is a term exclusively used for round shapes.👨🏻‍🏫

But this does not mean there's no baseline to build up other proportions in fancy diamonds. Instead, most proportions of fancy cut diamonds use the width as the baseline. For example, diamond width is the broadest measurement in a marquise-shaped diamond between two bellies. The total depth, table, and many other proportions of a marquise-shaped stone are calculated based on the diamond width.

Girdle diameter, diamond size, and diamond carat

In the context of the most-favored round diamonds, diamond size essentially equals average girdle diameter since the two dimensions of round shapes are nearly the same. Yet, this is not the case in the world of fancy cut diamonds. Their sizes mirror both length and width—and the different combinations of the two measurements, which blend into length-to-width ratios (L/W)—generate varying size perceptions for the wearers or observers.

When diamond carat is concerned, it is still mixed up with diamond size by many new diamond buyers.😞 Diamond carat denotes the weight of a diamond. Here, carat is a standard unit, and one carat translates to 200 milligrams. Nonetheless, due to the inconsistent cutting techniques and uneven proportions within diamonds, the weight distribution of diamonds can have numerous results. While two diamonds have the same girdle diameter or face-up size, their carat weights may not be equivalent.

Where can I find the average girdle diameter?

Table size, crown height, total depth, pavilion depth, and girdle thickness correlate closely with average girdle diameter. Gemologists and diamond graders use it to determine these values, which then help to assign the final cut grade for the gem.

As consumers, knowing the average girdle diameter helps us to perceive how large the sparkling gem is. We do not need to use a loupe to inspect like a pro. The jeweler and diamond grading report already did it for us!

Listing the measurements clearly on product pages is an obligation the FTC (Federal Trade Commission) requires. And we can also effortlessly find these measurements on diamond reports. But please note that the average girdle diameter will not show up as it is. Most diamond laboratories use the "width x length x height" format to merge the essential information. The first two measurements indicate the gem's minimum and maximum diameter for a round brilliant cut diamond. They're more than enough for you to understand the diamond's size. However, if you want to find out the average diameter, do the calculation yourself!

All the measurements matter!

We hope you now have a clear sense of the average girdle diameter. Though crucial, it is only one measurement in a diamond. All values, including percentages and angles, work with polishing, design, and face-up appearance to impact a diamond's cut quality.

To assist you in learning how each part of a diamond interacts with the others, check out this diamond anatomy guide. And don't miss this comprehensive post if you want to have a clear idea of diamond cuts.

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