Lower-half length is a proportion measurement, expressed as a percentage, that describes how far the lower girdle facets of a round brilliant cut diamond extend from the girdle toward the culet. It is one of the eight proportion parameters reported on the GIA Diamond Grading Report and one of the principal levers a cutter uses to tune the optical appearance of a finished stone. In the context of fancy shapes such as ovals, pears, and marquises, the same term is used by analogy to describe the length of the corresponding pavilion halves.

Definition and measurement

The lower-half length is the ratio of the distance from the girdle to the tip of the lower girdle facet, divided by the total distance from the girdle to the culet, expressed as a percentage. A value of 75 percent indicates that the lower girdle facet extends three-quarters of the way to the culet; a value of 80 percent indicates a longer, narrower facet. GIA reports the value rounded to the nearest five percent. Lower-half length is measured optically by reflectance scanners such as the Sarine DiaMension or by manual interpretation of the pavilion image.

Effect on appearance

The most readily visible consequence of lower-half length is the apparent thickness of the eight arrows seen through the table when the stone is viewed face-up under a hearts-and-arrows scope. Shorter lower-halves, in the range of 70 to 75 percent, produce broad, bold arrows and stronger black-and-white contrast. Longer lower-halves, around 80 to 85 percent, produce slim, elongated arrows and a softer, more diffuse pattern. The trade-off is between the strong on-and-off scintillation favoured by some buyers (especially in larger sizes) and the finer, more uniform sparkle preferred by others. Both ends of the range are compatible with GIA's Excellent cut grade, given suitable pavilion angles and crown geometry.

Cutting practice

The proportion is established when the lower girdle facets are polished after the pavilion mains. A skilled cutter will hold the lower-half lengths within roughly two percentage points across all sixteen facets to preserve optical symmetry. Inconsistent lower-half lengths show as uneven arrows and broken contrast in ASET and IdealScope imaging, and are penalised in symmetry grading even when other proportions are sound. Many ideal-cut programs (such as the AGS Ideal-cut classification before the lab's transition to GIA in 2022) specify a narrower lower-half-length range, typically 75 to 80 percent.

Application beyond rounds

In fancy shapes the term is extended to describe the proportional length of the pavilion halves running from girdle toward the culet. The optical consequences are similar: shorter halves produce greater contrast and a more compartmentalised pattern, while longer halves produce stretched, narrower returns. In ovals and pears, lower-half lengths are often deliberately varied along the length of the stone to balance the central bow-tie effect.