|GRADING OF MOUNTED DIAMONDS|
| Grading of mounted diamonds involves the following parameters.
In many cases, a stone can only be observed from the table side because of its margins hidden by mounting. The defects located in the marginal zones of the stone and those only visible from the pavilion side sometimes cannot be detected. Hence, high clarity grades are not assigned to mounted diamonds. Mounted diamonds up to 0.29 ct in weight are usually evaluated at no higher than grade 3, or exceptionally at grade 2, when a stone is easily available for observation. Mounted diamonds more than 0.30 ct in weight are evaluated at no higher than grade 4 (rarely grade 3). Within the international grading system, the highest clarity grade for mounted diamonds is VVS2.
Mounting significantly affects the color perception. A stone can be darkened or lightened, depending on the mounting type and color of the metal. In such cases, color grading can be conducted without masterstones and standard paper sheets. It is difficult to find a similar mounting for masterstones, while the paper influences the metal tint. During the color grading, the optimal position of the evaluated stone should be found so that the influence of the metal color on the perception of the stone color was the least. The color is evaluated under the standard light source; the stone is examined face up with a 10x loupe. As a rule, the true color of a stone can be seen in profile examination (from the girdle side). The color of the whole stone is evaluated. If the profile examination is impossible, the color grading is performed for the crown. A stone may be observed from any direction. Face-up grading often brings about significant errors. Visual grading without using masterstones is not so accurate; hence, the highest color grades are not assigned to the stones in this case. Diamonds up to 0.29 ct in weight are evaluated at no higher than grade 3 or, with rare exception, grade 2, when the major part of a stone is available for examination. Diamonds more than 0.30 ct in weight are evaluated at no higher than grade 4 (rarely, grade 3). Within the international grading system, the highest color grade for mounted diamonds is F or G.
According to the rules currently adopted in Russia, fancy color grades are not assigned to mounted diamonds. If the stone cannot be removed from its mounting, the last color grade is established.
The weight of mounted gemstones is determined through weight-estimation formulas (see "Estimation of Diamond Weigth" topic). These formulas allow weight estimation with an accuracy of 2 to 10%, if careful measurements are possible. Accuracy more than 10% is not satisfactory. The error increases for larger stones, resulting in too optimistic values. Applying a loupe with micrometric scale to diamonds more than 3.5 mm in diameter results in a small error in measurements and, therefore, provides more precise weight estimation. When it is impossible to measure the girdle thickness of round diamonds, the weight correction can be derived from the ratio of height to diameter (H/D). If the H/D rati๎ ranges within 0.57-0.66, no correction is needed for weight estimation. If this ratio is less than 0.57, the weight deduction may range up to 5% (the lower the H/D ratio, the higher the percentage of the correction). If the H/D ratio is higher than 0.66, the weight premium up to 5% may be required (the higher the ratio, the higher the percentage of the correction).
The most reliable identification of a mounted diamond can be performed with a diamond detector, which differentiates it from all imitations, except for moissanite. When the needle of the diamond detector hits metal, old detectors without metal indicator, may show error. In this case, the tool responds to the metal in the same way as to diamond. Visual differentiation of diamond from its imitations is usually difficult, especially for the stones with emerald, baguette, and incomplete cuts (Single Cut, Swiss cut). Other methods are described in "Diamond Identification" topic.
Proportions of mounted diamonds can be evaluated only by eye. In many cases, such examinations are missed, exept for large diamonds with significant symmetry features (for example, old-cut diamonds) and stones available for examination in all directions.
Price deductions and additions
In addition to the price corrections established for cutting features (see Russian Price Sheets topic), price deductions established for external features may be required for mounted diamonds (Price List No. 111):
Diamonds with small ships on culet and edges and other minor mechanic defects are appraised with a deduction of 20%.
Diamonds with significant chips and symmetry features, which can only be removed by recutting, or a crack reaching the table surface are appraised with a deduction of 40%.
Diamonds which do not meet the TU requirements are appraised with a deduction of 50%.
When mounted diamonds are described, stones with a similar weight and similar features may be grouped, if they belong to a single size-weight category. The description should include cutting style, the number of evaluated diamonds, the total estimated weight, and color and clarity features. Diamonds belonging to one size-weight category, but having different color and clarity characteristics are described separately. Nonstandard cuts, e.g. old cuts with significant deviations from the round shape or with a very large culet, are not specified in the description, but the number of facets should be indicated. "Rose" and other old cuts having less than 57 facets are described in the same way as Single Cut, but appraised with a deduction of 50%.