Experience has shown that the color of diamonds can be very exactly graded by comparison with master stones. As a good set of comparison stones form the most important necessity for reliable color classification, the relevant requirements of such a comparison set are set out as follows:
a) The comparison set should represent all color stages
of the yellow series.
In order to guarantee an even and gradual progression from colorless to yellow, all the brilliants in the comparison set should be in the same position within their color grade, e.g. all stones should lie within the upper; middle, or lower zone of the relevant color-grade.
The most advantageous is the lower zone, for all stones which appear only a "touch" darker are classified in a lower grade. The grader should know precisely, the position of his comparison stones In the color zone under consideration (Fig 2).
Fig 2 Ideal gradual color comparison series in the lower color zone
b) The comparison stones should have a minimum size of OAO ct. Within this size brilliants up to one carat can easily be graded. But for anyone frequently grading larger diamonds, comparison stones in the sizes from 0.70 ct upwards are recommended.
c) The comparison diamonds need not be flawless. But any inclusions present should not affect the color of the stone (e.g. brownish cracks or colored mineral inclusions).
d) Obviously fluorescent diamonds which exhibit varying colors in day and electric light should not be used for comparison purposes. The 'color should be constant in all types of light.
e) The comparison brilliants should be similarly cut and well proportioned, for with a crown or pavilion cut too high or too low, the light reflections emerge at different angles from those in the ideally cut brilliant, and could make color-grading more difficult. Thus brilliants with deep pavilions look darker, ones with shallow pavilions appear lighter than a well proportioned brilliant.
f) The girdles of brilliants should be as narrow and even as possible. Thick and fringed girdles could affect the color determination in a negative way, as part of the light is lost through the girdle. A reduced brilliance results from this. In bruted girdles fragments of the corn tongs and dust are in time deposited; the comparison stones should be boiled occasionally, otherwise their color impact changes.
g) Brown, grey and green tinted brilliants are not suitable for a comparison set based on yellow color.
In order to avoid confusion with the stones to be graded it is advisable to prepare an identity card for each of the comparison stones, on which its weight clarity-grade with inclusion picture, and measurements are entered.
To ensure their identity, but also for practical use in color-grading it is advisable to get the natural girdles of the comparison stones faceted or polished half-way round. Thus, according to the type of girdle of the brilliant being graded, the comparison stone with its bruted, faceted or polished girdle can b turned into the direction of sight of the observer, thus rendering a better comparison possible. The IDC, however, prescribes natural, bruted girdles on the comparison stones.
For storage the accessory trade provides practical boxes with compartments for the various color comparison stones. Their most important identifying features can be noted on the lid or the inside.
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