The girdle is the outermost edge which dictates the shape of the diamond. Unmagnified, it may appear to be a thin line, ;but is often a small series of facets. Grading reports cite type and describe thinnest to thickest points as verbal desriptors or percentages (described below).
1. Girdle Types
A faceted girdle is one where the brillianteer polished facets into the diamond's girdle.
An unfaceted, or bruted girdle is one where the diamond's girdle is unpolished and has a frosted appearance. Some cutters believe polishing the girdle may cause color to be reflected back inside, so unfaceted girdles are common in J or K color grades and down, and in small goods, .25 and lower. In near-colorless diamonds it's not the concern that it is with slightly colored and below.
A polished girdle on a diamond is see-through or clear. A polished girdle is considered a finishing touch but is not critical. One situation where polish is desirable is when the diamond is a fish-eye, where polishing will reduce the effect of the girdle's reflection in the table.
New machines have been developed that are putting a fine finish on girdles. These diamond girdles are more than bruted, but less than polished. We choose to call these finely finished: It is not faceted, not completely polished, nor does it remain bruted. It is in a warm and cozy place somewhere in-between.
For example, this EOS uses a liquid-cooled scaife to cool the diamond being bruted. It’s connected to a computer monitor that shows a zoomed image and a wire frame model that can be adjusted to the desired shape, has automatic steering and allows no vibrations during operation. The result is an extremely round, straight and high quality girdle.
Extremely Thin girdles on diamonds are usually avoided because of durability issues.
Very Thin girdles on diamonds are frowned upon as averages, particularly in princess cuts, but a portion of a diamond's girdle ranging to Very Thin may not be problematic as long as it is not the overall average.
Thin, Medium and Slightly Thick are preferable averages for diamond girdles.
Thick, Very Thick and Extremely Thick girdles on a diamond 'hide' weight, cause the diamond to appear smaller than it should for its carat weight
, and can adversely influence light performance (some proprietary 'clipped corner' cuts are an exception, as thick corners are by-products of their design).
3. Average Thickness
Girdle thickness is reported from min to max and commonly spans one or two descriptions (Thin-Medium or Thin-Slightly Thick for instance). The average is what is most significant. If a girdle runs from V Thin to Thin, with an average of thin, there may be just one microscopic portion of the girdle at V Thin with no deduction or durability issue. If there is a question it's prudent to consult a trusted professional who has the diamond in-hand.
As a visual aid, here are DiamCalc software-generated average girdle thickness comparisons.
American Gem Society's (AGS) Girdle Thickness Chart
(Copyright, AGS 2005, used with permission)
4. AGS and GIA Reporting
Note: AGS and GIA
girdle reporting is treated differently. The AGS notes thickest and thinnest places at any point on the girdle. GIA reports the thickest and thinnest parts measured at valleys only. Therefore girdle notations on AGS reports will seem to have much more variation.