| |
Composite
table of symmetry distortion grading
Table 2. Composite table of symmetry distortion grading defined
in different grading systems (allowable deviations from the ideal
shape, at which the symmetry is still considered as good) for a
round brilliant cut diamond with a weight of 0.50 ct. Major deviations
are marked in bold.
| Distortion |
Allowable magnitude |
| TU
(Russia) |
HRD
(Belgium) |
GIA
(USA) |
| Out-of-round girdle |
0.1 mm |
<= 4% |
cannot be noticed with
the naked eye |
| Table displacement |
1,5% |
< 4% |
observable through
a 10x loupe, not striking |
| Culet displacement |
1,5% |
< 4% |
observable through
a 10x loupe, not striking |
| Table inclination |
not defined |
crown height variation
< 4% |
observable through
a 10x loupe, not striking |
| Wavy girdle |
cannot be noticed with
a 10x loupe |
crown height variation
< 4% |
cannot be noticed with
a 10x loupe |
| Crown and pavilion displacement |
10% of girdle facet base |
allowed |
observable through a 10x loupe,
not striking |
| Pointing error |
cannot be noticed with a 10x
loupe |
allowed |
observable through a 10x loupe,
not striking |
| Improper shape of facets |
cannot be noticed with a 10x
loupe |
allowed |
observable through a 10x loupe,
not striking |
| Non-uniform girdle |
25% of minimum thickness |
allowed |
observable through a 10x loupe,
not striking |
| Additional facets |
<= 4 (on the pavilion) |
allowed |
a few |
| Unequal angles |
<= 1° |
---------- |
---------- |
Thus, examining the three diamond grading systems reveals both
community of the approaches they use and some differences between
them. It is worth noting that the concept of symmetry does exist
in all the systems but is never defined clearly in any of these.
Moreover, different systems differently assign symmetry distortions
to either proportioning or finishing. In certain cases, special
procedures of quantitative measurements are defined, but in some
other cases they may be lacking. The Russian grading system takes
into account deviations of the angles of crown and pavilion facets,
the HRD system takes into account deviations of the crown and pavilion
heights, while the GIA system does not consider these deviations
as symmetry distortions.
|
|
