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Glossary
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To locate the proportions that influence the face-up appearance of a round brilliant diamond and to learn more about how GIA measures them, click on the proportions listed below.

Diameter
Distance measured between two opposing points along the girdle's outline. The minimum and maximum girdle diameters are determined by taking a number of diameter measurements. Diameter is reported to the nearest hundredth of a millimeter (0.01 mm).

Average diameter is used in calculating certain percentages such as table size, total depth, crown height, and pavilion depth.

Average diameter = (min diameter + max diameter) ÷ 2

Total Depth %
Total depth (measured from table plain to culet) relative to the average diameter reported to the nearest tenth of a percentage point (0.1%).

Total depth % = (total depth ÷ avg diameter) x 100

Table Size %
Avg table size relative to the avg diameter, reported to the nearest whole percentage point (1%). Table size is measured from bezel point to bezel point; the average of four measurements is used.

Table % = (avg table size ÷ avg diameter) x 100

Crown Angle
Average of all eight crown angles, reported to the nearest half of a degree (0.5°). A crown angle is the angle of the bezel facet plane relative to the table plane.

Crown Height %
Average crown height relative to the average diameter, reported to the nearest half of a percentage point (0.5%). Crown height is measured from the table plane to the intersection of the bezel facet with the girdle.

Crown height % = (avg crown height ÷ avg diameter) x 100

Pavilion Angle
Average of all eight pavilion angles, reported to the nearest multiple of 0.2° (e.g., 41.0°, 41.4°). A pavilion angle is the angle of the main facet plane relative to the table plane.

Pavilion Depth %
Average pavilion depth relative to the average diameter, reported to the nearest half of a percentage point (0.5%). Pavilion depth is measured from the culet to the intersection of the pavilion main facet with the girdle.

Pavilion depth % = (avg pavilion depth ÷ avg diameter) x 100

Star Length %
The horizontally projected distance from the point of the star facet to the edge of the table, relative to the distance between the table edge and the girdle edge. This is averaged for all 8 star facets and reported to the nearest five percent (5%).

Lower-Half Length %
The horizontally projected distance from the point where two pavilion mains meet to the closest edge of the girdle, relative to the distance between the girdle edge and the center of the culet. This is averaged for all 8 lower-half facets and reported to the nearest five percent (5%).
Girdle Thickness (verbal description)
Described and reported by the GIA Laboratory as a range from the thinnest to the thickest "valley" areas (i.e., "thin places" located between the bezel-main intersection and where the upper and lower halves meet). Descriptions include extremely thin, very thin, thin, medium, slightly thick, thick, very thick, and extremely thick. "Extremely thin" appears as a knife-edge; that is, the crown meets the pavilion with no girdle in between.
Culet Size (verbal description)
Described and reported by the GIA Laboratory as the average width of the culet relative to the average diameter of the girdle. Descriptions include none, very small, small, medium, slightly large, large, very large, and extremely large. "None" is used when there is no culet facet, which is sometimes referred to by the trade as a pointed culet.