Gemologists must often evaluate diamond symmetry. For a rigorous evaluation, you’ll need a proportion scope. This device lets you project the image of the diamond onto a screen to measure lengths and angles. 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.

An Inexpensive Alternative to a Proportion Scope

During the 1970s, Dr. Kazumi Okuda solved these problems. 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.

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.

If the plastic reflector is red, the red zones mean a perfect return of the light. The white…