While you can measure girdle thickness and express it as a percentage of average girdle diameter, most people judge it by eye. Much of its influence depends on the size of the stone. A thick girdle can create unattractive, large, fuzzy, gray reflections in the stone. If it’s too thick, it can make the entire stone look darker. On the other hand, a thick girdle might actually enhance the face-up color of a fancy-color diamond.
Uniformity is another consideration. On a step cut or a rectangular brilliant, the girdle should be the same thickness all the way around. On a round, oval, or cushion-shaped brilliant, it should be slightly thinner between pairs of mains and girdle facets, and thicker where the points of crown and pavilion main facets meet.
Extreme variations in girdle thickness can cause problems when the stone is set. But sometimes the variations are intentional. The girdles of marquises, pears, and hearts are often slightly thicker at the points to reduce the possibility of chipping. Hearts have thicker girdles in the clefts, too.
The girdle on a diamond can be unpolished, which is called a bruted girdle. A bruted girdle should look frosted or waxy. If the girdle-shaping process is done incorrectly, the girdle will have a rough, granular surface. A rough girdle can trap oil and dirt, and it can eventually get so dirty that the stone will look dark.
The girdle might also be polished or faceted. A polished girdle has a smooth surface, while a faceted girdle has a series of smooth, flat facets. Polishing or faceting a thick girdle can make it less obvious, and also make the girdle reflections brighter. When you judge girdle thickness, it doesn’t matter whether the girdle is bruted, polished, or faceted.