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Chromium-diffused corundum.

Recently, two stones were submitted to the American Gemological Laboratories that provide a reminder for the potential of old treatments to reappear in the marketplace. The two purple-pink gems weighed 0.83 and 0.95 ct (Figure 27). The client had purchased them as part of a larger acquisition of various gem varieties, from a trader selling old stock that had been in his inventory for more than 20 years.

Microscopic observation of both stones revealed traits consistent with pink sapphires that had been heated (Figure 28), including zones of partially dissolved rutile inclusions, heavily altered crystals and partially healed fissures with a frosted appearance. In addition, their surfaces were heavily pitted (Figure 29). When viewed face-up with the unaided eye, they showed relatively even coloration, yet with magnification they had an atypical patchy appearance. Viewed in immersion, they displayed the distinctive uneven coloration from one facet to another that is associated with diffusion treatment (Figure 30). These observations, coupled with relatively high concentrations of chromium (~2.5 wt.% [Cr.sub.2] [O.sub.3]) recorded with EDXRF spectroscopy, confirmed that the two samples consisted of natural corundum that had been subjected to Cr-diffusion treatment. The EDXRF analyses also showed traces of Fe, Ti and Ga in both stones (Table II).

Refractive index readings of both stones revealed uncharacteristically high values (Table II), together with indistinct shadow edges at ~1.76 and 1.77 corresponding to typical corundum values. The high RIs are due to the enriched Cr content near the surface resulting from the diffusion treatment, while the typical values are produced from the underlying corundum where the Cr had not penetrated. Additional gemmological properties collected from these samples are listed in Table II.

Although the client had presumed these sapphires had been heated, he was very surprised to learn that they had been diffusion treated, as he was unaware that Cr-diffusion treatment even existed. It has been many years since Cr-diffused natural corundum was first reported (McClure et al., 1993), and this treatment was short-lived due to the apparent difficulty of diffusing Cr. Further attempts at Cr diffusion resulted in the inadvertent overgrowth of synthetic ruby onto a natural corundum host (Smith, 2002). Although both of these treatment processes were done on a limited basis, a small number of these gems did make their way into the gemstone market.

It has been more than two decades since the author has encountered Cr-diffused natural corundum, and the present stones stand out as a reminder that such uncommon products may be recycled back into the gem market at any time. Therefore gemmologists, appraisers and jewellers must remain aware of not only what is currently taking place in terms of treatments and synthetics, but also what has been done in the past.

Christopher P Smith FGA (

American Gemological Laboratories

New York, New York, USA


McClure S.F., Kammerling R.C. and Fritsch E., 1993. Update on diffusion-treated corundum: Red and other colors. Gems & Gemology, 29(1), 16-28,

Smith C.P., 2002. "Diffusion ruby" proves to be synthetic ruby overgrowth on natural corundum. Gems & Gemology, 38(3), 240-248,

Table II: Gemmological properties of the Cr-diffused
natural corundum samples.

Weight                        0.95 ct              0.83 ct

Refractive indices       Indistinct shadow edges: 1.76 and 1.77
(table facet)
                          More distinct:       More distinct:
                          1.780 and 1.788      1.772 and 1.780

Optic character          Uniaxial negative

UV fluorescence
Long-wave                              Strong red
Short-wave                        Weak red with chalky
                                   blue-white patches

Visible spectrum              Standard chromium absorption

Mid-infrared                 Weak 3309            Weak 3309
spectroscopy                [cm.sup.-1]          [cm.sup.-1]
bonded OH groups)                               Nominal 3232

                             Cr: 2.39             Cr: 2.59
                             Fe: 0.08             Fe: 0.02
EDXRF spectroscopy *         Ti: 0.03             Ti: 0.14
                             Ga: 0.01             Ga: 0.01

* EDXRF values are calculated as approximate wt.% oxides.
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Title Annotation:Gem Notes: TREATMENTS
Author:Smith, Christopher P.
Publication:The Journal of Gemmology
Date:Jun 1, 2015
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