The stone is positioned in the standard orientation, that is, with its table horizontal and facing downwards. One or several topmost facets (that is, those touching the topmost point) deviating from horizontal by no more than 20° are considered the culet facets.

If there are additional facets or caverns located no farther from the topmost point than 5% of diameter and touching the culet, they are considered a part of it.

Culet diameters are measured in 360 directions with the step of 0.5 degrees (because opposite directions apply to the same diameter). The diameter of the culet in some given direction is measured as a distance between two parallel vertical planes placed perpendicularly to the given direction as close to each other as possible, so that all vertexes of the culet lay between these planes (similarly to usual diameter). The minimum, maximum and average culet diameters are calculated using this array of values.

Culet length

In case of step cuts, the culet diameters are not very informative. If the culet contains no facets, the diameters are reported as 0. On the other hand, there might be a thin elongated facet which is considered a part of the culet.

To obtain a more robust characteristic, culet length is considered. Its measurement includes the culet facets (if any) and the culet edge. To locate this edge, the stone is divided in halves, and the topmost point of each half is found. The edge connecting those is the culet edge.