In our light-performance scores, symmetry measures how evenly the pavilion facets of each angle family are cut. It reads 100 when every facet in a family has the same angle and distance from the center.
In more detail
It is not the symmetry grade on a grading report. A report grade judges how exactly a real stone's facets are shaped, placed and aligned. Our score reads the cut's geometry only: a multi-angle pavilion counts as design, not as asymmetry. The Hearts & Arrows Scope gives a third, separate symmetry figure.
How we measure it
We group the pavilion facets into families. A new family starts where consecutive angles differ by more than 0.45°. For each family, we take the standard deviation of its angles, in degrees. We add 57.3 × the relative standard deviation of the facets' distances from the center. σ is the size-weighted mean of that sum, and Y = max(0, 100 − 6σ). It is the same in every gemstone.
Related terms
- Symmetry: Symmetry is the report grade, from Excellent to Poor, for the exactness of a stone's outline. It also covers the shape, placement and alignment of the facets.
- Hearts and arrows symmetry: Hearts and arrows symmetry is the share of a hearts and arrows pattern where all sixteen halves match. Our Hearts & Arrows Scope gives it as a whole percent for each view.
- Overall score: The overall score is one figure out of 100 that combines our light-performance scores for one cut in one gemstone. The Cut Performance Wheel calls it Global.
- Cut Performance Wheel: The Cut Performance Wheel is a free Digital Gemology tool. It draws eight light-performance scores of one cut, in one gemstone, on a wheel.
- Mirror-image symmetry (design): Mirror-image symmetry in a faceting design is the number of identical mirrored sections around the stone. An 8-fold mirror-image design repeats the same pair of mirrored halves eight times.