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The girdle is the narrow section of the diamond separating the crown from the pavilion and functions as the diamond’s setting edge as well as the blunt surface reducing the risk of damage from a blow to that edge. The girdle is measured in relative thickness and the type of finish.

Years ago, diamond girdles were left unfinished after the diamond was bruted, where diamonds are ground against each other to create the basic shape. This left the girdle looking dull and rough in appearance. Today’s gem quality diamonds are faceted with lots of very small flat facets or they are polished smooth. The faceted girdle makes it more transparent and thus more desirable.

The girdle of the standard round brilliant diamond has an undulating pattern, comprised of sixteen “valley” and sixteen “hill” positions. There is usually some variation in girdle thickness around the stone. The girdle thickness can be measured using a variety of optical measuring devices and expressed as a percentage relative to the average diameter. However, visual observations are the deciding method for assigning the thickness ratings.

The GIA uses relative thickness ratings for the girdle that range from Extremely Thin to Extremely Thick and uses the “valley” points as their thickness measures.

  • Extremely Thin
  • Very Thin
  • Thin
  • Medium
  • Slightly Thick
  • Thick
  • Very Thick
  • Extremely Thick

The AGS shows the girdle range as percentages of the diamond’s width and uses the “junction” points, which are the “hill” points as their thickness measures. The following table shows how the new AGS Girdle grades relate to the girdle thickness percentage and descriptions.

Insert table

Often there are four thinner areas on a diamond reflecting the original shape of the octahedral rough diamond crystal. If the girdle is Extremely Thin in these areas, the diamond could be prone to chipping or bruising. Diamonds with Extremely Thick girdles weigh more for their size and may not be as bright as they should be. The bigger girdles are also more visible, especially if they are not faceted.

Warped Diamonds

Beware of jewelers who mention the term “warped diamond,” especially if they use that term to describe a diamond you might be considering from some other retailer. Author, Fred Cuellar, introduced the warped concept in a book when he stated – If the average girdle thickness plus the average crown height plus the average pavilion depth does not add up to the total depth, then the diamond is “warped”. Interestingly, almost every diamond any retailer has to sell is “warped” by his definition while diamonds he is selling are not “warped”. If this sounds like scam to you, join the rest of the diamond industry.

The reality is that the difference between the measurements has to do with out averages are calculated by the grading laboratories and in particular, how the girdle is measured.

For example, here is an example of a diamond showing the average and ranges of values determined by a Sarin machine.

60.6% Total depth percentage

15.6% Crown height percentage – Average 15.2% (14.9-15.6)

+1.7% Girdle thickness percentage – Average 1.4% (1.0-1.7)

+42.4% Pavilion depth percentage – Average 42.3% (41.8-42.4)

=59.7% Total of percentages

Since the depth percentage is more than 0.5% different from the total of the percentages, Mr. Cuellar contends the diamond is “warped”.

The rest of the diamond industry simply points out that Sarin machines takes thickness of the girdle measurements at the valley position. To make the numbers add up, the girdle measurements who have to be taken at the bezel thickness. As long as the GIA and other grading laboratories use the valley position for girdle measurement, the numbers for most of the diamonds in the world will not have numbers that add up exactly.

We have had several clients have jewelers tell them that our diamonds are too warped to be set in a mounting but the jeweler could find them diamonds that are not warped. We just encourage our clients to search online for the terms “warped diamonds” and they quickly discover that these jewelers, along Fred Cuellar, are using a scare tactic to prey on unsuspecting diamond shoppers.

There are no “warped” diamonds and anyone using this term to dissuade you from purchasing a diamond is using a fraudulent tactic, so the sooner you exit their store the better.