The girdle is the edge of the diamond and is quoted in either words (Thin, Thick etc) or as a percentage of the diameter (e.g. thin = .6% or thick = 3.0%).
Bruting is the rounding process of grinding two diamonds against each other on a lathe that results in a ground glass like finish. Today larger diamonds mostly have faceted girdles (nearly 100 tiny facets), or they may be polished shiny all the way around. A bruted girdle does not lower a diamonds grade, and only looks worse if the stone is a 'fish-eye'.
The best girdle thickness is Thin, Medium or Slightly Thick. There is almost always some variation in girdle thickness around a stone; Thin to Slightly Thick etc. Sometimes there up to four small thinner areas that may include remnants of the original (squarish octahedral) rough diamond crystal. The remnants are not a concern if they are confined to the upper part of the pavilion facets. They may be described on a certificate as ‘naturals’ and marked in green on the report plot. If the girdle has Extremely Thin or Very Thin areas it is best not set them in exposed areas in rings; thin parts can be cleaved or chipped by a blow. These stones are OK in pendants and earrings, but there is still a very slight risk of a girdle chipping during setting.
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.
Be wary when selecting diamonds that have pointy ends with Extremely, Very Thin, or even Thin parts on the points because there is a high risk of damage during prong setting. The example in the photo shows a dangerously thin point on a Princess cut.
In USA girdle thickness of rounded diamonds were always measured at the thinnest part or 'valley'. In Europe the girdle is measured at the main or bezel facet junction, which is normally 1.6 or 1.7% thicker than the valley. The new AGS system now measures girdle thickness at the thick part.
Warped diamonds. Mr. Fred Cuellar wrote a diamond buying guide and has advised consumers not to buy diamonds if the crown, girdle and pavilion percentages do not add up to the same as the total depth percentage; he calls these "warped diamonds". Therefore most US graded diamonds are warped by 1.6 or 1.7%. Hopefully GIA will also change and then there will be no more ‘warped’ diamonds. e.g. 15% crown height, 1% Medium girdle and 43% pavilion depth should have a 60.7% total depth, not 59%. The girdle measurement at the thickest part would be 2.7%. 15 + 2.7 + 43 = 60.7%
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Very thick, medium, and very thin girdle