Gem News International Gems & Gemology, Fall 2019, Vol. 55, No. 3

Imitation Opal with Interesting Play-of-Color Pattern

Imitation opal blocks display play-of-color phenomenon.
Figure 1. This opal imitation shows a billowy play-of-color phenomenon. The largest block measures approximately 61.79 mm in length. Photo by Robison McMurtry and Diego Sanchez. Courtesy of Sanwa Pearl & Gems Ltd.

GIA’s Carlsbad laboratory recently examined samples of a relatively new manufactured gem material sometimes known as a “hybrid opal product” (figure 1) (see Spring 2018 Lab Notes, pp. 60–62). This material showed a refractive index of 1.498–1.500, and the specific gravity ranged from 1.33 to 1.35. Both of these properties fall far outside the range for natural opal, making identification very straightforward for the gemologist. These anomalous gemological properties also mean that this material is best described as an opal imitation and not a synthetic opal. This material is reportedly composed of about 20% silica and about 80% resin, which is consistent with the gemological properties measured.

Play-of-color phenomenon in imitation opal.
Figure 2. The imitation opal showed broad swaths of the play-of-color phenomenon when examined with a microscope. Photomicrograph by Nathan Renfro; field of view 18.80 mm.

This new imitation opal showed a pronounced play-of-color phenomenon with a pattern unlike the kind one would expect to see in conventional synthetic opal. Instead of small cellular patches, it displayed broad swaths of ever changing play-of-color covering large areas of the surface (figure 2). The laboratory examined samples that showed either a black or white bodycolor and a variety of colors in the play-of-color areas. For those who love opal, this attractive material could offer a cost-effective alternative to natural material while retaining the allure of the play-of-color phenomenon.

Nathan Renfro is manager of colored stones identification at GIA in Carlsbad, California.