Pavilion main facets

The pavilion main facets are the eight large kite-shaped facets on the bottom half of a round brilliant. They run from the girdle down to the culet (or the pavilion point) and define the pavilion angle — the most sensitive proportion in the cut grade. On a standard round brilliant the pavilion mains meet at the culet in a true point or a small culet facet; on Old European and Old Mine cuts they meet at a larger culet facet.

Pavilion mains drive total internal reflection. Incoming light enters through the table, refracts as it crosses the air-diamond boundary, hits a pavilion main, reflects across to the opposite pavilion main, and returns up through the crown to the eye. The geometry only works inside a narrow pavilion angle band — 40.6° to 41.0° on GIA Excellent rounds, 40.7° to 40.9° on AGS Ideal-0. Outside that band light leaks through the pavilion (shallow) or returns at the wrong angle (deep) and the stone faces up dim.

The pavilion mains share their geometry with the lower-half facets (sometimes called lower girdle facets), the sixteen smaller facets that sit between adjacent pavilion mains and run from the girdle partway down toward the culet. The lower-half length is the proportion that controls scintillation pattern around the perimeter; AGS Ideal-0 rounds run 75% to 82% lower-half length. A stone with shorter lower-halves shows broader flashes; longer lower-halves produce finer, crushed-ice scintillation.

Pavilion main symmetry is a load-bearing input to the GIA Symmetry grade. The eight mains should be identical in size, shape, and angle; the angle deviation between mains is one of the metrics GIA measures when grading Symmetry as Excellent vs Very Good. AGS Ideal-0 stones run pavilion main symmetry tight enough that the hearts-and-arrows pattern is visible under a standard viewer — the eight arrowheads visible face-up are the eight pavilion mains reflected through the crown.

The pavilion mains terminate at the culet (or at the pavilion point if there is no culet facet). Two adjacent pavilion mains can meet poorly — leaving a small offset where they should converge — and the GIA Symmetry grade flags this as "pavilion mains not meeting at point." On AGS Light Performance grading the same defect shows up as a visible asymmetry in the ASET image.