The girdle of a diamond refers to its outermost edge and serves two basic functions; to prevent damage and to provide a surface for setting the stone. In the GIA grading lab, the girdle is examined and described based on its thinnest and thickest points.
For example, you might find a description like (medium – slightly thick) or (thin – thick) in a grading report. Technical note: the terms thin and thick are defined as a percentage of its diameter, thin being .6% or thick being 3%.
This means that for a larger sized 2 carat diamond, a girdle thickness of thin would be physically larger when compared to a 0.5 carat diamond with a girdle thickness of thin.
What You Need to Know About Variations in Girdle Thickness
Unlike round diamonds where the parameters of girdle thickness play a role in determining cut grade, it isn’t the case for fancy cuts. In fact, slightly thicker girdles can help protect vulnerable edges and corners by providing additional mechanical strength.
That said, this works like a double edged sword. The drawback of a thick girdle is that it makes the diamond look smaller since the weight is hidden in the profile view.
To illustrate this, I am going to show you a comparison of a 0.90 carat stone (medium – thick) against a 1.01 carat stone (thick – extremely thick). Feel free to click the links in order to view the full details of these 2 stones.
The 1.01 diamond on the left measures up at 5.86*6.59*4.21 mm while the 0.90 carats on the right measures up at 6.09*6.66*3.86 mm. This means that the smaller carat diamond LOOKS BIGGER than the heavier stone!
In this example, the cutters deliberately saved weight on the 1.01 to hit the “magical one carat” mark so that it sells for more. It shouldn’t come as a surprise that the 0.90 carat is priced at $4,130 while the 1.010 carat costs $4,710.
Let me reveal the ugly truth of diamond cutting to you now. The majority (>90%) of fancy shaped stones are cut to retain weight and maximize profits for the jeweler/cutter. This is often done at the expense of achieving better cut quality.
As a general guideline, the maximum girdle thickness I would recommend when buying a diamond is “very thick” as this will help you avoid unnecessary “dead weight”. I would always avoid diamonds that have a uniform “extremely thick” girdle because they represent poor value and are often cut to bad proportions.
Don’t Go Too Thin Or You Have Heightened Risks of Chipping
To maximize spread and value, some people think that choosing diamonds with thin girdles is a smart move. However, I’m sorry to say that this is a terrible idea since there are durability risks involved. An analogy I would use is the “Goldilocks” concept – where too much or little of something isn’t ideal.
As a guideline, I recommend choosing a diamond with a girdle range between thin and thick. Yet, there are no hard and fast rules for the upper boundary. For example, I would still consider diamonds with girdle thickness of “slightly thick” or “very thick” provided the price is right and the stone exhibits superb light performance.
However, I would avoid stones with mentions of very thin or extremely thin girdles since they are prone to chipping in that localized area.
With a knife edge at the lobes, this diamond is disaster waiting to happen.
Does the Finishing of the Girdle Matter?
Technically speaking, the gemological labs don’t take the type of girdle finishing into account when evaluating a diamond (except when it is a severe case of a bearded girdle). If a cutter decides to go a step further and create a polished or faceted girdle, this does not improve or degrade results in the grading process.
A polished or faceted girdle is purely a choice of personal preference and will not affect the optical performance of the diamond. In my opinion, I find faceted girdles to be the most appealing as it provides a complete look. If you are interested, you can read up more about girdle finishing here.
On the next page, I will describe a step-by-step process that you can use to select a great looking fancy shape diamond…