Lab Notes Gems & Gemology, Spring 2018, Vol. 54, No. 1

New Plastic Imitation Opal from Kyocera

Nathan Renfro and James E. Shigley

Plastic opal imitation by Kyocera alongside the company's polymer-impregnated synthetic opal.
Figure 1. This new plastic opal imitation (left) shows a very pleasing play-of-color that does not have the classic "snakeskin" columnar structure one would expect to see in traditional polymer-impregnated synthetic opal produced by Kyocera (right). The largest sample of plastic imitation opal in the left photo weighs 227 grams, and the largest sample of synthetic opal in the right photo weighs 65 grams. The rectangular black block in the photo on the left is 9.7 cm in length. Photos by Robert Weldon.

Recently, the Carlsbad laboratory examined an interesting new gem material manufactured by the Japanese company Kyocera that displayed a play-of-color phenomenon. In the trade, this material may be sold as an “opal-like product,” because of the play-of-color phenomenon observed (figure 1, left).

Standard gemological testing revealed a hydrostatic specific gravity (SG) of 1.35. The refractive index (RI) was measured as 1.49. According to GIA, an imitation is defined as a natural or manmade material that mimics the appearance of, and is used as a substitute for, another gem material. Likewise, a synthetic gem is defined by GIA as a laboratory-grown gem material with virtually the same chemical and physical properties as a natural gem. Because the gemological properties for this new material are out of range for natural opal (SG of 2.00 and RI between 1.37 and 1.47), it is best described as an imitation opal.

IR spectroscopy shows that the plastic opal imitation's ~1734 cm-1 peak is about five times as strong as the synthetic opal's feature.
Figure 2. The infrared absorption spectrum of the new plastic imitation opal (red trace) was very similar to that of traditional plastic-impregnated synthetic opal (blue trace) except that the polymer feature at ~1734 cm–1 was about five times as strong.
Further testing revealed that this material readily burns with a hot point and produces a strong acrid odor indicating that a significant amount of plastic is present. Additionally, advanced testing using energy-dispersive X-ray fluorescence (EDXRF) spectroscopy revealed the presence of silicon. Fourier-transform infrared (FTIR) testing revealed a spectrum that was quite similar to traditional polymer-impregnated synthetic opal (figure 1, right) except for a polymer peak that was nearly five times as strong (figure 2). These tests indicate that this material is composed of both plastic resin and silicon. When exposed to ultraviolet light, samples with a white bodycolor fluoresced a weak blue to short-wave and medium blue to long-wave UV. Samples with a black bodycolor fluoresced a weak orange to short-wave UV and were inert to long-wave UV.

Patterns in synthetic opal that are not seen in the new product.
Figure 3. Traditional synthetic opal shows a "chicken wire" or "snakeskin" pattern (left) and a columnar structure (right) that are diagnostic of synthetic origin. The new plastic opal imitation does not show these features. Photomicrographs by Nathan Renfro; fields of view 7.20 mm (left) and 14.40 mm (right).
Play-of-color patches are uniformly distributed throughout the plastic opal imitation.
Figure 4. Randomly oriented patches of play-of-color are uniformly distributed throughout this new plastic opal imitation. The imitation looks the same from all different angles. Photomicrograph by Nathan Renfro; field of view 13 mm.
Microscopic examination revealed some interesting features. The pattern in the play-of-color was random and unlike traditional synthetic opal did not display any “snakeskin” or “chicken wire” pattern (figure 3, left), but instead showed colored polygonal patches about 2 mm in size uniformly distributed throughout (figure 4). Nor did the plastic imitation opal display the columnar structure one would expect to see in synthetic opal (figure 3, right).

Visually, this new material is a very good imitation of natural opal. Gemologically, it is easy to separate from natural opal due to the comparatively high refractive index and low specific gravity. There are other possible decorative applications for this plastic opal imitation material due to the large sizes that can be produced. This attractive material is a welcome addition to the gem trade and is easy to separate from natural opal with standard gemological testing.

 

Nathan Renfro is the manager of the gem identification department and microscopist in the inclusion research department, and James E. Shigley is distinguished research fellow, at GIA in Carlsbad, California.