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Gemologists must often evaluate diamond symmetry, and a rigorous evaluation requires a proportion scope. This device projects the image of the diamond onto a screen, so lengths and angles can be measured. Unfortunately, this device has two drawbacks. First, it’s designed primarily for evaluating loose stones. Second, it’s quite expensive. As a result, many gemology students try to evaluate the symmetry of a set diamond without a proportion scope. They’ll take a risk and do a visual and imprecise “loupe and eye” evaluation.

Fortunately, there’s an effective, alternative tool you can use. I’ll show you how to build it yourself.

By Dr. Raul Berenguel, PhD. 3 minutes read
solitary ring

18K white gold solitaire ring and a 1-ct brilliant-cut diamond, before setting. Photo by Mauro Cateb. Licensed under CC By-SA 4.0.

An Inexpensive Solution

During the 1970s, Dr. Kazumi Okuda created an alternative to the proportion scope. He utilized 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 - plastic reflector

Figure 1

The interior of the plastic reflector reflects light from a lamp to the diamond. The gem reflects that light, now red or reddish, with either a perfect or imperfect shape, depending on the diamond’s cut and proportions.

If the diamond is perfect, the observed image will be identical to Figure 2.

figure 2 - diamond symmetry

Figure 2

If the plastic reflector is red, the red zones mean a perfect return of the light. The white zones indicate a leak of the light, which means a flaw in the proportions. The triangular black forms allow you to evaluate the symmetry. A pink (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 - diamond symmetry

Figure 3

You should classify the example above as “Poor,” assuming symmetry categories of “Excellent,” “Very Good,” “Good,” “Fair,” and “Poor.”

How to Test Mounted and Loose Diamonds

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

A Do-It-Yourself Way to Measure Diamond Symmetry

If you have a triplet lens laying about unused, you can build one of these tools easily. For a reflector, choose a tubular piece of any red plastic with a flat interior surface that reflects light well. (I successfully used a red cover of WD-40 lubricant. See Figure 4). The ideal material shouldn’t be totally opaque. Your tubular piece can have a constant diameter or an enlarged one like a funnel. The only assembly to be done is uniting the piece of plastic to the lens. Remember that cyanoacrylate glues don’t mix with optic parts.

figure 4 - diamond symmetry

Figure 4

For the area between the lenses and the cover, use any cylindrical piece of material on hand. (You could also ask a machinist to create an aluminum cylinder). In the closed part of the cover, cleanly cut a circular hole 6 mm in diameter exactly at the center. This size diameter will allow you to fully see diamonds up to 1 carat. You can see how the lens and cylinder go together in Figures 5 and 6 below.

figure 5 - diamond symmetry

Figure 5

figure 6 - diamond symmetry

Figure 6

Remember, the opening of the red reflector should be at the focal point of the lenses. If you’re unsure about this, just focus the image from a lamp with the lens over a piece of paper. With the image at its minimum, carefully measure the distance between the lens and the paper.

For very accurate diamond symmetry measurements, you can use a filar micrometer with a goniometer. See Figure 7.

figure 7 - filar micrometer and goniometer

Figure 7