Unraveling an opal's Play of Color
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  Unraveling an opal's Play of Color

by Guy Borenstein, FGA EGG
December 13, 2012

One of the trickiest gems to evaluate is an opal. This beautiful gem displays a distinctive phenomenon of colored flashes, which reflect from the stone while it is rotated. The effect is called "Play of Color," and it is a term that well explains the experience.

The visual effect of the opal is a result of its physical structure, which involves silica spheres of about 150 to 300 nanometers in diameter in hexagonal or cubic close-packed lattices. They produce the internal colors through the interference and diffraction of light passing through the stone. As the opal is turned, the light is affected differently, and so is created the "Play of Color."

While most gems are graded using the classical factors - color, clarity, weight, etc. - Play of Color is an additional factor that must be considered with opal. How is this done?

The quality of the Play of Color depends on several characteristics. These can be the color of the flashes, their dispersion over the gem, the patterns they produce, and more. It is complicated to determine and demands an essential quality in the grader - experience.

In order to visualize and better understand this phenomenon, this month we decided to use the GemePro™ Sampler to inspect the Play of Color effect of an opal gem. Since the Sampler was designed to define the body color of gems, we didn't actually know how it would deal with the challenge this unique effect posed. However, we believed we could get useful information from the flashes that would be of assistance in understanding its grading method.

For this research, we received from L. Allen Brown of All That Glitters (http://www.atggems.com) images of a magnificent 8.89-ct. oval cabochon-shaped Ethiopian opal (Figure 1), which was described as displaying a "massive" play of color effect, in a pin fire pattern. The gem was a good candidate, as it shows the effect with several different colors distributed all over the dome.

We used a face-up image of the opal and scanned it using the Sampler. Instead of retrieving an average body color (which is useless if the flashes are part of the calculation), we recalibrated the Sampler sensitivity to be as twice sensitive as it regularly is. By doing that, we were able to provide the main color groups and their ratios within the overall appearance of the gem. From the resultant list, we eliminated the body color hues and were left only with the colored flashes.

As seen in Figure 2, the main colors that the sampler identified were Red (5.88%), Orange (0.37%), Yellow (0.37%), Green (1.48%), Blue (1.48%) and violet (0.37%).

The results demonstrate the visibility of the effect and its intensity - almost 10% of the gem reflects Play of Color flashes. Considering the fact that the analysis was carried out on a static image, the amount of flashes in reality may even be higher. Moreover, we can see that the effect is not limited to a few colors, but rather their combinations cover the entire spectrum.

Looking at the Color DNA (Figure 2, left hand side), it is clearly evident that the colored reflections are distributed all over, which means that they are spread over the entire dome. This beautiful opal has indeed earned its play of color effect's description as "massive."

If you have a magnificent gemstone or colored diamond and would like Gemewizard® to analyze it in one of its next Gem Color Reports, please contact us at info@gemewizard.com.