Girdle thickness

The girdle is the narrow band of a faceted diamond where the crown (top) meets the pavilion (bottom). It is the part of the stone the setting grips. GIA grades girdle thickness on an eight-step descriptive scale: Extremely Thin, Very Thin, Thin, Medium, Slightly Thick, Thick, Very Thick, Extremely Thick. The visible range on most well-cut rounds is Thin to Slightly Thick. AGS Ideal-0 rounds run Thin to Slightly Thick; outside Very Thin to Thick the cut grade drops a tier.

Girdle thickness matters for two reasons: durability and weight. Extremely Thin girdles chip at the setting under normal wear — a four-prong solitaire on a daily-wear ring puts point loads on the girdle exactly where the cutter left no material to absorb them. GIA flags Extremely Thin in the comments section because the durability risk is real; insurers and bench jewelers downgrade stones with Extremely Thin girdles. The opposite end of the scale — Extremely Thick — is durable but hides weight. An Extremely Thick girdle adds material between the crown and pavilion that contributes to carat weight without contributing to face-up size; a 1.05 ct round with an Extremely Thick girdle can face up like a 0.95 ct.

The girdle is either bruted (matte, frosted surface from the older cutting process), polished (smooth, mirror finish), or faceted (cut as a series of small facets running around the perimeter). Modern cutters use faceted girdles on most premium stones because the facets catch and return light at the perimeter; bruted girdles are still common on commercial-grade rounds. Polished girdles sit between the two on light performance.

The girdle is the canonical location for the laser inscription. GIA inscribes the report number on the girdle of every graded diamond — the inscription is what ties the physical stone to the paper. The location ensures the inscription does not affect the table's appearance and can be verified by any retailer with a microscope. AGS Ideal Report supplements and IGI lab-grown reports use the same convention.

Girdle thickness varies around the stone. GIA grades the minimum and maximum thickness measured at the eight bezel positions; a "Thin to Slightly Thick" girdle is normal variation. A "Thin to Thick" girdle indicates poor symmetry — the stone is not cut to consistent tolerances and the cut grade reflects it.