The lower girdle facets are the sixteen narrow, triangular facets that occupy the upper portion of the pavilion on a standard round brilliant-cut diamond, running between the girdle edge and the eight pavilion mains. They are the pavilion counterpart to the upper girdle facets on the crown, and together with the pavilion mains they shape light return from below the table. In a textbook fifty-eight-facet round brilliant the lower girdle facets are the most numerous on the pavilion and the most influential on the appearance of contrast at the table.

Geometry and proportion

Each lower girdle facet is bounded by the girdle, by two adjacent main pavilion facets meeting along the kite-shaped pavilion main, and by the neighbouring lower girdle facets above. The proportion most often discussed by graders is the lower-half length, expressed as a percentage of the distance from the girdle to the culet that the lower girdle facet occupies. Common ranges run from about 70 percent (short, robust virtual facets) to 85 percent (long, thin lines and an open look). GIA's cut grading reports lower-half length to the nearest 5 percent.

Optical role

Lower girdle facets generate the narrow virtual facets that fan from the girdle toward the centre, producing what is sometimes described as the arrows pattern in well-cut hearts-and-arrows stones. The length of the lower halves controls the apparent thickness of these arrows: shorter lower-half lengths produce broader, blockier arrows and a stronger contrast pattern, while longer lower-halves produce slim, elongated arrows with a more delicate look. Although both can score the highest cut grade, perception of fire and scintillation shifts subtly between the two ends of the range.

Cutting and finish

The lower girdle facets are typically among the last to be polished on the pavilion, after the pavilion mains have been seated and the culet (or culet point) is established. Polishing the sixteen lower halves uniformly is a measure of the cutter's hand: subtle differences in length or angle along the row are detectable in optical-symmetry imaging such as ASET and IdealScope viewers. Pavilion-main angles in the 40.6 to 41.0 degree band combine with disciplined lower-half lengths to produce the brightest, most uniformly contrasted reflection patterns.

Beyond the round brilliant

Equivalents of the lower girdle facet appear on most modified brilliant cuts, including the cushion modified brilliant, the oval, the pear, and the marquise. In each case the count and proportion are tailored to the outline of the stone, and lower-half length remains a critical lever for cutters seeking even light return across an asymmetrical shape. In step cuts such as the emerald cut, the equivalent role is played by the lower step facets rather than by triangular halves.