Diamond symmetry refers to how precisely the various facets of a diamond align and intersect. Do the edges of the facets on the top and bottom of the diamond line up along the girdle? Are the facets aligned symmetrically around the diamond?
This can include extra or misshapen facets, off center culets and tables, and wavy girdles.
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.
The GIA symmetry grade categories are; Excellent, very good, good, fair, poor, very poor.