The table of a cut diamond has a very important influence on the brilliance. As the largest "window" of the brilliant it has the task not only of refracting as much of the incident fight as possible collectively into the stone; but also through the table, the greatest amount of light totally reflected on the pavilion facets must be emitted to the eye of. the observer. In order to fulfill these requirements the table must be cut as large as possible. But that would mean ignoring the crown facets and, in particular, the girdle facets - function of dispersing the light refracted back out of the stone (ct. Fig 190 on page 161).
It is therefore important to create an optimal ratio between maximal light emission through the table and the greatest possible dispersion through the girdle facets.
In this connection brilliants cut in the old style are once more recalled. They are distinguished by a small table and steep crown facets and therefore show a lively play of spectral colors; on the other hand, the light yield remains poor as a result of the small table. In contrast to this the Parker brilliant attains a high output of light through its large table but, because of its shallow crown facets, emits hardly any dispersion, and such stones look "tired". The ideal is a table size of about 56 to 62 % of the girdle diameter.
|Fig 207 Gauge for measuring size of table||Fig 208 The table is measured in at least two directions corner to corner|
In recent years practice has shown that many diamonds found on the market exhibit considerable deviations of the table size and the crown height. The table sizes often measure 68-70% of the girdle diameter. This method of cutting is related to commercial considerations, as with a large table the loss in weight is about 10% less. The brilliant also looks bigger than a brilliant of ideal cut, but the dispersion of such a stone is insufficient.
For the exact measurement of the table .size small table gauges with millimeter subdivisions are available (Fig 207); with these the distance between two opposite "corners" of the table is measured under the microscope, at 10 to 20 fold magnification according to the size of the stone (Fig 208). For this the zero-point of the measuring gauge is "laid at one corner and the figure where the opposite table corner cuts the millimeter scale is read off.
But should the measurement of the table size be made from table edge. to table edge, 4 % must be added to this result to allow for the sh6rter distance.
Since the table is not always absolutely uniformly cut, it is advisable to measure it in four different directions. The larger value is then calculated as a percentage of the girdle diameter, using the following formula:
table size X 100
girdle diameter (average value)
With a little practical experience and practice the table size can very easily be estimated within ± 1 % by visual estimation. This can be very helpful especially when buying diamonds. when no instrumental aids are to hand .
In order to practice estimating the table size, the grader should always first estimate the table measurement visually. There are-two very simple methods of gauging the table, which are described in detail in the following paragraphs.
|Girdle diameter||Table ratio measurement|