Symmetry refers to how precisely the various facets of a diamond align and intersect. Quality is graded on the appearance of extra or misshapen facets, off-center culets and tables, and wavy girdles. Sometimes, a diamond cutter will deliberately allow a minor reduction in symmetry as a way of preventing a defect present in the rough stone. Symmetry typically ranges from 'poor' to 'excellent'.
Symmetry has a limited impact on the appearance of a diamond and a greater impact on price. A diamond with 'excellent' symmetry and polish may be priced higher than a diamond with 'good', 'fair' or 'poor,' because they are perceived as more valuable. A diamond with a lower symmetry grade will also have a higher probability of noticeable defects.
Each Reputable jewellers's diamond certificate displays the symmetry grade, using the following scale:
||No symmetry defects visible under 10x magnification.
||Any defects are very difficult to see under 10x magnification.
||Any defects are difficult to see under 10x magnification.
||Defects are noticeable at 10x magnification, and may be visible to the unaided eye.
||Defects are visible to the unaided eye.
A diamond with poor symmetry may misdirect light that travels into the diamond, sending it off at slightly wrong angles, and thereby reducing the diamond's brilliance. Often, a diamond cutter will purposefully allow a minor reduction in symmetry as a way of preventing a defect present in the rough stone from being retained as part of the finished diamond.
For diamonds with a symmetry grade of Excellent to Good, symmetry should not be used as a primary factor in choosing a diamond, since each of these grades is possible in diamonds of exceptional appearance.
Symmetry is more important in diamonds of VVS2 Clarity and higher, since the very subtle defects produced by Fair or Poor symmetry (which can resemble pinpoint inclusions) would compromise the diamond's otherwise flawless appearance.
Despite its modest impact on appearance, symmetry has a significant impact on price; a diamond with Excellent Symmetry and Polish may be priced 10%-15% higher than a diamond with Good Symmetry and Polish. This premium has more to do with consumer's perceived value of "excellent" grades, than the actual effect on a diamond's appearance.
Because diamonds with Poor symmetry have defects visible to the naked eye, these diamonds should be avoided in all cases.
A diamond should not be chosen or rejected based solely on symmetry. Because the overall cut grade already incorporates symmetry, it should be used as the primary determinant when choosing a diamond. When comparing two diamonds of equal cut grade, symmetry (and polish) can then be used as a further refinement or tie breaker.