Grading the symmetry of round brilliant cut diamonds at HRD (Belgium)

Consider now a symmetry grading approach adopted in Europe, for example, the one developed at HRD (Hoge Raad voor Diamant, that is, Diamond High Council), Belgium. When grading the cut quality of a diamond, three proportion groups (very good, good, and unusual) and four finishing groups (very good, good, medium, and poor) are used. Possible distortions of symmetry mostly affect the finishing grading, as well as exterior features, such as polishing errors, do (HRD, 1997).
To grade the finishing, the following rules are used:
- Very Good – flaws are very hardly observable with the naked eye, that is, observable only after their location is somehow determined,
flaws are hardly findable using a 10x loupe or microscope, that is, become conspicuous only after they are localized.
- Good – flaws are hard to find with the naked eye; they are not conspicuous, but clearly visible after they are somehow detected,
flaws are fairly easy to find, can be noticed, but do not strike one’s eye.
- Medium – flaws are easy to find with the naked eye,
they are striking when a 10x loupe is used,
they affect the brilliance of the stone.
- Poor – symmetry deviations are very striking and hindering, that is, are evident,
asymmetry distinctly affects the brilliance of the stone.

Four types of symmetry distortions are considered as the key ones. These are: deviation of the girdle shape from circular (out–of–round), variation of crown height or pavilion depth, off-axis displacement of the culet (culet’s eccentricity), and off-axis displacement of the table (table’s eccentricity) . The girdle non-roundness is measured by a micrometer, while the rest of the distortions are revealed by means of a proportionscope or microscope. There are less important symmetry distortions as well, some of which may also affect the appearance of the stone. These are:
- wavy girdle, due to a decrease in the tilt angle of two opposite pavilion facets (this may cause the effect of local darkening, similar to the «bow-tie» effect peculiar to marquises and some other elongated fancy cuts);
- alternating girdle thickness;
- cone-shaped girdle;
- lower main facets do not align with upper main facets.
Additional facets and some other symmetry distortions are considered as minor, because they do not considerably affect the appearance of the stone and its perception by the observer.

If any of the key symmetry distortions is revealed, it should be measured and normalized to the average diameter of the girdle. Table 1 illustrates the relation between the values thus obtained and the resultant finishing group the stone belongs to: