Proportions
Symmetry
Symmetry is usually rated using the same grade system as the proportions. A diamond with excellent symmetry will be perfectly round (or square, for princess cut stones and asschers), have the table and culet aligned in the centre of the stone, and all the facets aligned with each other in all directions. Diamond shapes with a length-to-width ratio significantly over 1 (such as oval, marquise, emerald) do not need to be perfectly round or square of course, but they still need to have all the facets and points aligned, and be highly symmetric along the main axis.
Some of the most common problems with symmetry include facet junctions that do not meet at a single point, tables and culets that are not exactly in the centre of the diamonds, facets that have unequal size or are tilted at different angles, table that is tilted with respect to horizontal, upper and lower girdle facets that do not meet at the same position, girdle that is wavy and has non-uniform thickness. Only when everything is geometrically symmetrical can we expect the brilliance promised by the proportions of the stone, otherwise some of the rays will be reflected off of the wrong facet and will be directed through the pavilion rather than through the table and the crown and towards the eye of the observer.
The main reason for less than perfect symmetry is in the original shape and properties of the rough stone. When inclusions are present near the surface of the faceted stone, they will usually be cut away resulting in lower symmetry grade but better clarity. At the same time, depending on the shape of the rough, larger portions of the stone can be kept to increase weight, while somewhat sacrificing symmetry. Just like with proportions, this increased carat weight does not serve any purpose other than increasing the price of the stone and impairing its beauty. Occasionally, defects in symmetry are caused by lack of experience or care of the cutter. In general smaller, lower quality stones are more likely to be cut by less experienced cutters and are more prone to the imperfections in cut.
