Technically, crown height percentage is the value after the subtraction of pavilion depth and girdle thickness from the total depth of a diamond.
When calculated separately, crown height percentage is the mean value of the vertical distances from the table plane to the eight points where the bezel facets and the girdle intersect.
However, manually measuring the crown height percentage is unnecessary during modern diamond grading since the practice is complicated and inaccurate. Instead, certificated diamond graders generally get the estimated crown height percentage with either the help of visual computing—or by referencing the official crown height percentage table developed by GIA through table percentage and crown angle.
Simply put, crown height variation (abbreviated as CHV) indicates the inequality of the eight crown height values, causing the table plane to not be parallel to the girdle plane.
Because crown heights and bezel facets are connected, when there's crown height variation in a diamond's symmetry, misshapen bezel facets typically exist. (Nevertheless, misshapen bezels do not necessarily result in CHV.)
Moreover, the upper parts of hills (areas between the bezel and pavilion main facets) on the girdle of a CHV diamond are, as a result, not shaped evenly and distributed off balance.
For the reason that a consistent crown height lays the foundation for a regular octagonal table in a round brilliant diamond, crown height deviation probably leads to a non-octagonal table (T/oct) and table off-center (T/oc) when inspected face up.
It is common knowledge that the crown acts as the "windows" for the diamond to showcase its fascinating light rays and light play. Crown height variation has implications for the gem's appearance, optical properties and overall quality.