The similarities between the natural and synthetic sapphire markets extend beyond the parallels in quality range and pricing. Both natural and synthetic sapphire can be diffusion-treated to produce various effects and colors—in fact, colorless synthetic sapphire that has been diffusion-treated to induce a blue color is widely available (figure 4). Diffusion treatment of synthetic sapphire has been documented since the 1950s. As with natural sapphire, diffusion-treated synthetics are considerably less expensive than untreated specimens. Growth methods and treatments are not always disclosed, especially in less-expensive material. Nevertheless, diffusion-treated blue synthetic sapphire is easy to detect in an immersion cell filled with liquid of matching refractive index; the treated gems display color concentrations along facet junctions and the girdle, as shown in figure 4 (bottom) and figure 5 (top right). Diffusion-treated blue synthetic sapphire is inferior to grown blue sapphire in that the treatment is very shallow, eliminating the possibility of recutting. The treated goods may also be less transparent, as the thin diffused surface may exhibit noticeably different optical qualities from the bulk of the cut specimen. Fortunately, easy and inexpensive detection methods reduce the potential for confusion when purchasing a synthetic sapphire gem.
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I believe we will see a dramatic increase in the production of diffusion-treated blue synthetic sapphire originating from colorless material—a trend that could bring down the price of higher-quality synthetic blue sapphire. This is reminiscent of the concerns over beryllium-diffused natural sapphire a decade ago, when the market was flooded with pinkish orange padparadscha sapphire. We could start seeing much more of this material in a range of colors, given the large volume of high-purity and very clean commodity-priced colorless sapphire being produced. In the future, I would like to see full disclosure of growth technique and treatment for all synthetic gems, as having such information is of value to the consumer.
