If one looks at a brilliant at right angles to the table surface one sees in the culet a light to dark grey reflection of the table, which is surrounded by darker reflections of the table facets (Fig 246). Such reflections are most clearly visible with dark-field lighting if the light falls into the stone through the pavilion. In symmetrically cut brilliants the table reflection forms an even circle, which lies in a plane parallel to the table surface.
From the size of the table reflection in comparison with the size of the whole table of the brilliant, one can deduce the depth of the pavilion of the stone:
the deeper the pavilion of a brilliant,
the bigger is the table reflection in comparison with the actual table size,
and vice versa.
The reflection itself becomes darker with its increase. A brilliant with a shallow pavilion has for this reason a small and light table reflection, a brilliant with a deep pavilion has a large and dark table reflection (Fig 247 -250).
The ratio table reflection: pavilion depth can be very easily recalled, for ten units in the percentage of table reflection correspond to one unit of pavilion depth. From this the following table results:
|Table reflection in % age of table diameter||Pavilion depth in % age of girdle diameter|
|Fig 247 Brilliant with ideal
depth of pavilion = very good brilliance.
Reflection of table 20-30%
|Fig 248 Brilliant with 45% depth of pavilion = limit for "very good" brilliance. Reflection of table 50%|
|Fig 249 Brilliant with too large
a pavilion = medium brilliance.
Reflection oft table 60-70%
In an ideally cut brilliant the pavilion depth amounts to 43.1 %, the table reflection is therefore 30 % of the table size and light, which produces very good brilliance (Fig 247).
The limit for very good proportions is reached with a table reflection of 50 %, for with a pavilion depth of 45 % the cut lies at the extreme limit of tolerance for "very good" (Fig 248 and 253).
If the dark table reflection extends over the whole field of the table and leaves a light reflection only on the outermost edge of the table, the pavilion depth amounts to about 49 %. If the dark reflection also stretches on to the table facets, the pavilion depth may be taken as over 50 %. In this case all light rays have already been refracted by the pavilion facets and not totally reflected back into the crown. The brilliance is therefore strongly diminished.
Brilliants with a pavilion depth of 40 % or less show a light circle beneath the table which is caused by the reflection of the girdle. These flat brilliants are called "fish eyes" . The smaller the depth of the pavilion, the larger the reflection of the girdle in the centre of the table.
|Depth of pavilion||Table reflection expressed in percent age of the size of the table|
|Reflection of girdle visible under the table
Reflection of girdle visible at external rim of table.
Reflection of girdle visible just outside the table
|Reflection of table a little less than ideal
Reflection of table about a third of table area (light grey field) = ideal.
Reflection of table just a little larger than ideal (1/3)
Reflection of table about 1/2 of table area (limit of tolerance for "very good" cut)
|Reflection of table about 2/3 of table area (grey field) Reflection of table over 3/4 of table area (limit of tolerance for "good" cut)|
|Fig250 Estimating the depth of the pavilion by observing the table reflection||48 %
|Reflection of table covers nearly all of the
Reflection of table covers the whole table area (Cu1: poor) (dark grey)
Reflection of table infringes on table facets
From the position of the table reflections, deductions can also be made about the symmetry of the brilliant. If these reflections and mirror-images are irregularly and patchily distributed around the culet, it indicates that the brilliant has been cut not completely round but slightly oval, or that similar facets are not exactly the same size, or that they are not exactly opposite one another (Fig 262 and 263). If the table reflection does not surround the culet as an even circle, but is displaced to one side (Fig 264 and 265), then either the table or the culet is eccentric. In a brilliant whose pavilion is too shallow on only one side, this ·is perceptible as a semi-circular image of the girdle (Fig 266).
|Irregularly distributed reflection caused by unsymmetrical cut.|
Although these methods for estimating the pavilion depth may at first seem difficult. with sufficient practice and experience they permit the pavilion depth to be quickly gauged within an accuracy of 1 %.
Beginners are urged to practice all the methods described for visually Judging the proportions on every diamond and. to check his or her estimate by subsequent instrumental measurement of the proportions.
Cut Quality of Proportions
|Sideways reflection to estimate pavilion depth||Symmetry Features|