How Important is the Diamond’s Girdle?

Thin? Thick? Which do I prefer?

When looking at details of a diamond, either on the certificate or just what is listed on a diamond website, you will notice that one of the parameters is called “Girdle” and the options of what it might be measured at are: Thin, Thick, Medium and variations of those three.

What is the girdle, and what does the measurement mean?

The Girdle is the flat edge on the circumference along the edge of the diamond. It is the section of diamond between the Crown above and the Pavilion below. The measurement of the Girdle is quoted in either words (Thin, Thick etc) or as a percentage of the diameter (e.g. thin =.6% or thick = 3.0%).

The Girdle is formed by a process called Bruting. Bruting, as Wikipedia explains, “is the process whereby two diamonds are set onto spinning axles turning in opposite directions, which are then set to grind against each other to shape each diamond into a round shape. This can also be known as girdling”.

Bruting is basically the process of grinding two diamonds against each other on a machine called a lathe that results in a ground glass like finish. When Girdling a diamond, one can finish it by creating a faceted Girdle, by polishing the Girdle to a smooth and shiny surface, or by leaving it unfinished and dull.

Most larger diamonds today have faceted girdles – a faceted Girdle will generally have nearly 100 tiny facets!

A bruted girdle does not lower a diamonds grade, as it does not affect the appearance or quality of the diamond. The only time the Girdle might affect the diamond in any way, either appearance or price, is if the stone is a ‘fish-eye diamond’.

A fish-eye is a nasty appearance that you can see inside the Table of the diamond. It looks slightly crazed and dull, just like a dead fish’s eye. The fish-eye is a reflection of the girdle. If the girdle is not polished and is thick, the effect looks like a large rounded inclusion.

Fish-eyes are more apparent if the Pavilion is shallow, if the Table is large, and the Girdle is thick and not polished. Combinations of these factors worsen the effect.

The best diamond girdle thickness is Thin, Medium or Slightly Thick. Rarely will the measurement be a straightforward “Thin” or “Thick”. More common is when it is a combination of the sizes – “Thin to Slightly Thick”, “Medium to Extremely Thick”, etc.

Sometimes there might remain some small thinner areas that may include parts of the original rough diamond material. As long as they are on the upper part of the Pavilion, these remnants will not affect the diamond. This will usually be marked on the diamond certificate as “Naturals”.

If the Girdle is Extremely Thin or Very Thin areas, it is best not set them in exposed areas in rings; thin parts can be cleaved or chipped. These stones are OK in pendants and earrings, but there is still a very slight risk of chipping during the setting of the diamond. Be careful when selecting diamonds that have pointy ends with Extremely, Very Thin, or even Thin parts on the points – there is a high risk of damage during prong setting on such stones.

Diamonds with Thick, Very Thick, or Extremely Thick girdles weigh considerably more, so the spread of a diamond is reduced. But thicker girdles have little impact on a diamond’s sparkle. If a diamond has an Extremely Thick Girdle, one must be careful to ensure that it was not left Extremely Thick for the sole purpose of adding weight to the diamond. There is no benefit to the added weight when it is in the Girdle.

If, for example, the diamond is just over a price threshold, such as it is 1.00 or 1.01 Carat, and is listed as having a Girdle that is Extremely Thick, you should consider that the cutter bruted the diamond with that size Girdle for the sole purpose of getting the diamond over the weight of 1.00 Carat. Had he left it as a Medium or Thin, perhaps it would have weighed.98 or even less, thus costing you less.

You do not want to pay more for a 1 Carat diamond just because of the added weight of the Girdle.

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