| ROUND BRILLIANT PROPORTIONS | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Proportions | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| A stone shows the maximum brilliance and fire only when its cutting proportions exactly correspond to those calculated from physical and optical laws.
According to Tolkowsky, the ideal diamond cut has the following proportions (% of stone diameter):
Table - 53%, Pavilion depth - 43.1%, Girdle thickness - 0.7%, Crown height - 16.2%, Crown angle (a) -34
1/2° ; Pavilion angle (b) - 403/4° .
Actual parameters always deviate from these calculated values, although a diamond-cutter tries to keep the proportions within certain limits. Keeping b (41°) close to the ideal value is one of the most important conditions, since this parameter controls the total internal reflection of diamond. For example, the table size may vary, ranging from 55 to 65%, the girdle thickness may range up to 5%, and a may vary between 30° and 40°. The variation limits of the cutting parameters may differ in different countries and depend on the stone size and requirements imposed by manufacturers and clients. |
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Table
size evaluation |
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The table size ranging within 56-62% of the stone diameter is considered ideal by most dealers. The table size can be measured directly with a transparent micrometric scale, proportionscope or loupe with a micrometric scale. The distance between two opposite vertexes of the table is measured. Thus, four measurements are possible (along the octagon diagonals). It is recommended to accomplish all the four measurements and then derive the mean value. The table size is calculated by dividing the mean table diameter by the girdle diameter and is expressed in percentage. In addition, there are two methods for rough estimation of table size :
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Crown
angle evaluation |
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The crown angle typically ranges from 30 to 40°. The most accurate estimation of the crown angle can be performed with a proportionscope. However, a rough estimation is needed in some cases, which can be accomplished through the following methods:
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Girdle
evaluation |
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Judging by the surface character, girdle can be bruted or faceted. Rough girdle is considered as a disadvantage of polish. The main evaluated parameter of the girdle is its thickness. Evaluation can be accomplished by three methods:
The average girdle thickness is the thickness value most frequently established for a given stone. Thin to slightly thick girdle is regarded optimal. The diagram in figure can be used for conversion of the percentage girdle thickness into qualitative characteristics and for calculation of weight correction factors. |
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Estimation
of pavilion depth |
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The pavilion depth is interrelated with the pavilion angle B. It is believed that any deviation of B from 41 O affects badly the stone appearance and deteriorates its brilliance and fire. Hence, international grading systems always deal with pavilion depth rather than pavilion angle. The Tolkowsky ideal diamond has a pavilion depth of 43.1%, however, that of actual stones may vary between 40% and 50%. With a loupe and tweezers, the pavilion depth can be roughly estimated from the size of table reflection in pavilion facets (see figure). The diamond is examined face-up at the right angle. The reflection of the table in pavilion facets appears as a dark fringe. For more effective picture, the stone should be held somewhat below its ordinary position and should be examined at a greater distance from the lamp and against light background. Quantitative grading is performed with master cards (figure). When using this method, one should take into account the geometric distortions of crown facets, which certainly affect the diameter of the table reflection. This can bring about erroneous estimates of the pavilion depth: the more significant the deviations of the crown proportions from ideal parameters, the greater the error. |
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Estimation
of culet size |
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Only recently cutting technologies has enabled making a pointed culet. Almost all old-cut diamonds have large culets, which on face-up examination, appear as dark spots. The dark spots are due to the loss of light which enters the diamond crown and does not undergo total internal reflection. Therefore, a good culet is that not visible by unaided eye. The following culet grades are used:
Very small, small, or medium culets are best suited for diamonds. If a diamond has pointed culet, "no culet" is often written in the worksheet. Culet is the most fragile part of a diamond, hence it can be chipped off or abraded in the course of cutting, transportation, or mounting of the stone. |
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Effect
of proportions on stone price |
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To derive a proportion grade and estimate the stone price, one should understand how proportions determine the appearance and appeal of diamond. Opinions differ; some purchasers prefer diamonds with a large table, others - those with a small table. Some diamonds are very attractive in spite of significant deviations from the Tolkowsky ideal diamond parameters. For this reason, many grading systems involve only measurement but not grading of stone proportions. Calculated parameters of some ideal diamonds are presented in the following table: (Source: Verena Pagel-Theisen "Diamond Grading ABC")
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