How To Evaluate Diamond Symmetry

Gemologists must often evaluate diamond symmetry. For a rigorous evaluation, you will need a device called a proportionscope, which allows you to project the image of the diamond onto a screen to measure lengths and angles. This device has two drawbacks. First, it’s designed primarily for evaluating loose stones. (To evaluate the symmetry of a set diamond without a proportionscope, many gemology students decide to take a risk and do a visual and imprecise “loupe and eye” evaluation).

Second, it’s quite expensive.

An Inexpensive Alternative to a Proportionscope

During the 1970s, Dr. Kazumi Okuda solved these problems by falling back upon a 10X magnifying glass and a reflector of colored plastic (red, reddish, or orange color). This idea is based on a simple principle, illustrated in Figure 1.

Figure 1.

Light from a lamp is reflected by the interior of the plastic reflector to the diamond, which reflects that light, now red or reddish, with either a perfect or imperfect shape, depending on the cut and proportions of the diamond. If the diamond is perfect, the observed image will be identical to Figure 2.

Figure 2.

If the reflector is red, the red zones mean a perfect return of the light. The white zones indicate a leak of the light, that is to say, a flaw in the proportions. The triangular black forms allow you to evaluate the symmetry. A pink (and not red) color indicates a partial return of the light, which means a minor flaw in the symmetry. Observe the flaws in Figure 3.

Figure 3.

The example above should be classified as “Poor,” (on a diamond symmetry scale of “Excellent,” “Very Good,” “Good,” “Fair,” and “Poor”).

The position of the light depends on whether the diamond is loose or mounted. If the diamond is loose, the light can be oriented directly behind it. If the diamond is mounted, you can use an incandescent light coming from above (as in Figure 1) — never direct — but reflected by a piece of white paper. With practice, an aspiring gemologist will be able to determine the optimal type of illumination.

A Do-It-Yourself Way to Measure Diamond Symmetry

If you have a triplet lens you’re not using, you can build one of these tools with ease. As a reflector, it’s enough to choose a tubular piece of any red plastic whose interior surface is flat and reflects light well. (I successfully used a red cover of WD-40 lubricant!). The ideal material should not be totally opaque. It can be of constant diameter or enlarged like a funnel. The only assembly to be done is to unite the piece of plastic to the lens. Remember that cyanoacrylate glues do not mix with optic parts.

For the area between the lenses and the cover, use any cylindrical piece of material on hand or ask a machinist to create an aluminum cylinder. In the closed part of the cover, cleanly cut a circular hole 6 mm (0.24 inches) in diameter exactly at the center. This size diameter will allow you to fully see diamonds up to 1 carat. Figures 4, 5 and 6 illustrate the assembly.

Figure 4.
Figure 5.
Figure 6.
Figure 6.

Remember that the opening of the red reflector should be at the focal point of the lenses. If you’re not sure about this distance, just focus the image from a lamp with the lens over a piece of paper. When the image is at its minimum, carefully measure the distance between the lens and the paper.

For very accurate diamond symmetry measurements, you can use another device, a filar micrometer with a goniometer, like the one shown in Figure 7.

Figure 7.
Figure 7.