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Axial
symmetry matrix
Table 5. Degree of mutual deviation of the axes of a round
brilliant cut diamond.
| |
Table axis |
Crown axis |
Girdle axis |
Pavilion axis |
| Table axis |
0 |
|
1 |
|
| Crown axis |
|
0 |
2 |
|
| Girdle axis |
|
|
0 |
|
| Pavilion axis |
|
|
3 |
0 |
In the above table, figures 1, 2, and 3 stay for those symmetry
distortions considered major in the diamond grading systems discussed.
Namely:
1 – displacement of the table;
2 – non-parallelism of the table and girdle;
3 – displacement of the culet.
After the measurement of the angles between the axes is over, the
corresponding cells of the matrix are filled in with the data obtained.
The analysis of such a matrix, first of all, allows one to reveal
the true nature of the asymmetry if, for example, three of the axes
coincide while the fourth is inclined. Second, the angles measured
provide the possibility of evaluating how such-and-such symmetry
distortion affects the appearance and optical characteristics of
the diamond. For example, 1° tilt of the table plane results in
approximately 2° deflection of a ray leaving the diamond (starting
from here, table-to-crown rays are considered; and in the case of
a crown-to-crown ray the difference is yet larger, because the tilt
of the table almost does not deflects a table-to-table ray). Meanwhile,
1° error in the mutual arrangement of opposite pavilion facets results
in 12-16° deflection of an output ray, which considerably distorts
the visible distribution of brighter and darker regions in the diamond.
Among the main symmetry distortions, two types can be distinguished:
the distortions of the first type are due to mutual deviations between
the axes of the main elements of the diamond, while the second is
due to asymmetry of individual facets belonging to the same main
element with respect to the axis of this element. To grade the asymmetry
described, it is not enough to simply measure all the possible cases
of deviation from the ideal symmetry. In addition, one needs to
evaluate the influence of each type of symmetry distortion on the
appearance and resultant perception of the diamond. For example,
variations of the angle between a pavilion facet and the pavilion
axis belong to the second type, but the influence of this distortion
on the optical characteristics of the diamond is stronger than that
of non-coincidence of the table and pavilion axes.
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