The Color-Grader is an instrument developed by the GIA
for determining the
color of diamonds. It is intended as a supplementary piece of equipment to the diamond microscope made by the same manufacturer, and can be fastened to the microscope stage. The Color-Grader consists of a white plastic base with a wedge-shaped channel in which the comparison brilliants, with their tables downwards, are laid in a row (Fig 10). An additional light source above the microscope casts diffuse white light filtered through a matt glass perpendicularly on to the Color-Grader. In order to be able to push the brilliant being graded and its comparison stone into the microscope's field of vision, the instrument is movable sideways. A milled ring nudges the plane of vision to and fro to make possible different directions of sight.
The Color-Grader permits a color test even under microscopic magnification, which can sometimes be very useful. On the other hand, a color test of stones of different sizes by 10 x magnification is rendered difficult by the different focusing required.
A further disadvantage of the Color-Grader is that the surroundings are not screened off, so that color influences from outside impair the color test. The illumination is worse than that of the Koloriskop. Different sized stones do not lie in one focal plane and therefore cannot be brought into a common sharp focus.