Pavilion depth is the vertical distance from the plane of the girdle to the bottom of the stone (the culet, where present, or the meeting point of the pavilion main facets where no culet has been polished). It is conventionally expressed as a percentage of the stone's girdle diameter (or width, in fancy shapes), and it is one of the principal cut-grade measurements on any laboratory grading report. Pavilion depth is closely related to pavilion angle — deeper pavilions are associated with steeper angles — but the two are reported separately because they capture different aspects of the geometry and because they vary in their consequences for light return.

Optimal ranges

For round brilliant diamonds, the GIA Excellent cut grade range for pavilion depth is approximately 42.5 per cent to 43.5 per cent of girdle diameter. The narrowness of the range reflects diamond's high refractive index and the consequence of even small departures from the optimum. For corundum (ruby and sapphire), the range is somewhat wider — approximately 42 per cent to 47 per cent for round brilliants, with deeper pavilions tolerated for paler material to compensate for lower colour saturation. Lower-RI materials (tourmaline, aquamarine, garnet) require shallower pavilions, with optimal depth percentages in the 40 to 43 per cent range. In step-cut and fancy shapes, the geometry is more complex but the principle holds: there is an optimal pavilion depth for each combination of refractive index, hue saturation, and stone outline.

Failure modes

Two cutting faults relate directly to pavilion depth. Shallow pavilions — depth percentages below the optimum — produce windowing, the same fault as pavilion angles too shallow: light enters through the crown and exits through the back without returning to the observer. The result is a transparent patch through which the underlying setting and finger are visible. Deep pavilions — depth percentages above the optimum — produce a different problem: weight retention without proportional brilliance. A stone with a too-deep pavilion may carry significant carat weight without the optical performance the carat weight implies, because the additional pavilion volume contributes to weight but not to light return. Deep pavilions also produce nail-head extinction (a dark central area in the stone where the pavilion's reflected light fails to clear the table).

Trade implications

Stones with pavilion depths near the optimum command premium prices for their carat weight; stones with shallow pavilions discount because of visible windowing; stones with deep pavilions discount because their carat weight overstates their visual size and brilliance. A common diamond-trade convention summarises this: the trade pays for visible diamond, not for hidden weight. A 1.50 ct stone with a 47 per cent pavilion depth (deep) may face the table dimensions of a typical 1.20 ct stone with a 43 per cent pavilion depth, and the market discounts accordingly. For coloured stones, where each piece is unique and recutting is more often dictated by colour-zoning considerations, pavilion depth is one of several factors weighed against colour outcome and material loss.

Measurement

Pavilion depth percentage is calculated as (pavilion depth ÷ girdle diameter) × 100. Pavilion depth itself is measured at the bench with a millimetre gauge or, for laboratory work, with non-contact optical scanners (Sarine, OGI). Girdle diameter is measured the same way. Both measurements appear on the laboratory grading document, and the calculated percentage is the standard reference value for cut-grade purposes. See also pavilion, pavilion angle, crown height, table percentage.

Further reading

• GIA Gems & Gemology — cut grade and proportion articles
• International Gem Society — pavilion depth reference
• Lotus Gemology — coloured stone proportion analysis