Judgment of cut is based, on the one hand, on the impression gained at first sight, in which beauty and brilliance are judged purely by personal standards. Thereafter the expert directs his attention, on the other hand, to the proportions of the cut form. Here, facet angles and proportions must be tested for accuracy. The grading of the quality of the cut is therefore always made up of two components:' a subjective opinion of the brilliance effect, and an objective judgment arrived at by checking the "make" and measuring the proportions.
Judging the quality of cut, whether very good, good, medium, or poor, presents problems, especially to the beginner, because the factors involved in cut are manifold and may occur in very different combinations and variations of proportions, symmetry, and external features. Cut grading demands sound experience as numerous individual factors have to be taken into account when considering the overall quality of the cut.
It is all the more important therefore for the grader to have an accurate knowledge of the proportions and their influence on the brilliance of a diamond, as well as to be aware of the kinds and different possibilities of symmetry faults and external features which may occur. Only then can the grader pass a sure verdict on the quality of cut.
For this reason cut-grading requires good practical experience, for only by taking into account the many single factors can the grader arrive at a comprehensive appraisal of cut quality. Within the otherwise comparable nomenclatures, there still exists at present disagreement as to whether external features should be included in clarity-grading, the clarity-grade being affected other than by the internal features; or whether they should be, as in practice hitherto, included in cut-grading, separately from the inclusions. A separate evaluation of the internal and external features seems more logical and better related to existing practice, especially as, in most cases, external features may be cut away with little loss of weight.
Should it happen that no evaluation of proportions is included in the practice of cut-grading, the ability to grade cut would become especially important, to enable one to determine the quality of cut and the value of the stone itself. For abandoning the evaluation of proportions could give rise in the future to a situation where a diamond of good or medium cut-grade is sold and bought at the same price as a very well cut stone.
External features and most of the Symmetry featur.es are determined during clarity-grading in the course of work by tenfold magnification; they are graded as "very good", "good", "medium" or "poor" according to their size and number, position of the diamond, as well as the degree of visibility and effect on the brilliance. The reduction in value associated with the individual quality grades is given in the table overleaf.
Extensive damage, such as fractures on the girdle or on the point of the stone, are judged according to the loss in weight which may be assumed if the feature were cut away. This approximate loss of weight is equated with a certain reduction in value.
External features are faithfully reproduced on an identity diagram in green, showing their size and position in the diamond. Symmetry features are not drawn in.
The proportions and measurable symmetry features are
separately determined by the shadow-image method, for example with the
Proportion Scope, or by visual estimation, if the grader is sufficiently
The overall evaluation of the cut quality of an individual diamond is formed from a cross-section of observed characteristics.
If the overall judgment of the cut quality of a given diamond were a purely theoretical procedure and not one related to practice. The cut of a stone, having a large culet but otherwise very well cut, would be graded as "medium"; but it should be evaluated and judged from all points of view. In most cases a small area of judgment remains open, which can only be filled by experience. On this latter, the "make" of a stone, that is, the manual skill of the cutter, has a decisive influence in the overall verdict.
The considerable increase in price for rough diamonds in recent years has resulted In more and more diamonds being cut "for weight" at the expense of the quality of their cut. In going this, the cutter attempts to keep the loss in weight as low as possible, without reference to proportions.
The normal loss of weight of an average rough diamond when
consideration is given-to good proportions, amounts to about 50 to 60 %.
If the stone is cut "for weight" the loss in weight is only about 40 %,
but the proportions lie outside the tolerances, and the quality of the
cut is only medium or poor.
The larger stone is only apparently more valuable, for its value is governed by the amount the stone would lose in weight if it were re-cut to good proportions.
Such a "weight cut" is usually to be found when a stone is near the 0.50/1.00 carats border and is most ugly. The loss in value compared to a well-cut stone can be 15-20%. If the proportions are not evaluated, the end user is the looser as he believes that he purchased a one-carat, but in reality bought a stone with the value of a 0.80-0.85 carats diamond. A precise proportion evaluation shows up such a weight cut at once.
The following is presented as an example (Fig 202).
Out of two similar rough stones one cutter cuts a correctly proportioned brilliant, which weighs 1 .00 ct. The other cutter works "for weight", his stone weighing 1.15 ct, but its proportions are only medium or poor. The value of the second, larger stone is only apparently .higher than that of the one-carat stone, . for if the stone weighing 1 .15 ct is re-cut to very good proportions, it loses about 20 % of its weight. It would then weigh only 0.90 ct. This theoretical weight is called "the corrected weight". On the basis of the latter the evaluation of the stone proceeds.
|Fig 202 a Proportions within tolerances cut: very good 1.00ct||Fig 202 b Brilliant cut "for weight" cut: poor 1.15 ct||Fig 202 c Evaluation: loss of weight by re-cutting '" about 20% "corrected weight": 0.90 ct|
Cut-grading requires good practical experience, since only the consideration of many individual factors together makes possible a comprehensive qualitative judgment.
When grading the cut, the loss of brilliance must be taken into account which a diamond suffers because of poor proportions, symmetry and finish. The starting point is always the perfect cut, with some tolerances allowable.
The quality of the brilliance is dependent on the proportions, the symmetry and the external features.
The factors which contribute to the proportions are: -
- girdle diameter
- table diameter
- total height
- crown height
- thickness of the girdle
- pavilion depth
- angle of the crown facets to the plane of the girdle
- angle of the pavilion facets to the plane of the girdle.
To arrive at a judgment of the proportions, the grader measures the deviation from the standard data of a perfect brilliant.
The greater the deviations are, the more 'is the brilliance affected, and the lower is the quality of the cut. However, a very slight departure from standard proportions does not enable any loss of brilliance to be discerned even by the specialist. It is therefore normal practice not to apply exclusively the standard data, but to' extend these upwards and downwards within narrow tolerances. Only when a brilliant deviates beyond these limits of tolerance can it no longer be graded "very good", but only either "good", "medium", or even "poor".
Testing of the symmetry features relates to:
- the position of the table and culet,
- the size and regularity of the facets, girdle and culet.
Symmetry features are in part measurable, or are observed by 10 x magnification and classified into the four cut-grades according to the degree of their visibility and of their effect on the brilliance of the stone.
External features comprise the following:
- naturals and extra facets
- twinning-, growth- and grain-lines
- fringed or rough girdle
- polishing marks
Comprehensive evaluation of cut characteristics based on the supplementary Draft for RAL 560 A.5 of 1970
1. Proportions 2. Symmetry 3. External features
|Grade||Definition for Diamond Brilliants over 0.5 ct.|
|very good||1. Proportions: Table size 52-64 %
Crown height 12-18 %
Pavilion depth 42-45 %
Girdle thickness: thin, i.e. less than 3 %
|2. Symmetry features: none or only visible with difficulty.
Very good brilliance.
|3. External features: none or very minor natural ones.|
|Good||1 . Proportions: The tolerances exceed by a maximum of 5
those permissible for "very good", e.g. Table size 68 %,
Crown height 11 %, Pavilion depth 47 % or less than 42 %,
Girdle thickness: medium to thick.
|2. Symmetry: isolated symmetry features, not too difficult
see. Slightly reduced brilliance.
|3. External features: a few fairly small.|
|Medium||1. Proportions: The tolerances exceed by a maximum of 10%
to those permissible for "very good'~, e.g. Table size 71 %,
Crown height 9 %, Pavilion depth 49 % or less than 40 %,
Girdle thickness: thick.
|2. Symmetry: several symmetry features, not too difficult
see. Brilliance perceptible reduced.
|3. External features: a few fairly large.|
|Poor||1. Proportions: show striking deviations. i.e. more than
of those permissible for "very good"; Girdle very thick.
|2. Symmetry: many easily discernible symmetry features.
Brilliance obviously reduced.
|3. External features: many and mostly large.|
|Summary table||Cut-grading of small brilliants|