Effects of gamma irradiation on ruby and pink sapphire: an update

By Dr. Hao Wang, first published in Facette 29 (May 2024)

Figure 1: Purple to pink sapphires before and after the so-called ‚hospital' irradiation treatment. As evident from the photo, the colour shift induced by the irradiation treatment varies from stone to stone from nearly no change to a rather distinct change. Photo: SSEF.

Gamma irradiation is a treatment method that may enhance the colour and visual appearance of gemstones, such as diamond, topaz, quartz, and corundum, to name a few. This treatment method uses strong gamma rays, either from radioactive isotopes like cobalt-60 (60Co) or from a linear accelerator (LINAC), to induce point defects in the crystal structure of gemstones. These defects, caused by the displacement or ionization of atoms, can lead to the creation and activation of so-called colour centres. This process can alter the gemstone’s colour in ways that range from subtle to dramatic, with the extent of the change depending on intrinsic properties of a gemstone (e.g. chemical composition and site occupancy) and the specifics of the irradiation process.

The exploration of gamma irradiation as a technique to enhance the colour of corundum dates back several decades. The advent of ruby lasers (synthetic, mostly of light colour and with pinkish hue) in the 1960s sparked extensive investigations in optical and material sciences into how gamma radiation affects these ruby lasers, especially its impact on their performance. In the 1980s, a keen interest appeared in the gem trade to experiment with gamma irradiation. The aim was to intensify the colour of yellow or pale gem-quality corundum and to bring these now more attractive irradiated stones to the market. It is thus safe to say that the irradiation of corundum is not a new treatment, but somehow remained mostly below the ‘radar’ of the gemmological community and the trade in the past decades, probably as the quantity of irradiated corundum was rather limited and mainly restricted to yellow sapphires.

The new so-called ‘hospital’ treatment

This has changed in the past few years, as we have multiple trustworthy sources which indicate that quite a number of rubies and fancy sapphires ranging in colour from pink to purple, light yellow and even blue may undergo an undisclosed gamma irradiation treatment before entering the marketplace (Figure 1). As this is a new challenge to the trade and labs alike, it initiated immediate research activities and discussions among gem labs with the aim to develop criteria to detect this irradiation treatment (Pardieu et al. 2022, Scarratt – pers. comm. 2022, Leelawatanasuk – pers. comm. 2022, Krzemnicki 2022, Krzemnicki 2023). Based on our information, this gamma irradiation treatment was originally carried out (secretly?) in medical facilities, utilizing cancer radiotherapy equipment, hence its name ‘hospital’ treatment. Based on more recent information, the treatment now is also carried out at non- medical irradiation facilities, mainly in countries known as cutting and trading hubs of corundum.

As mentioned above, the aim of this irradiation is to change and shift the initial colour to a more vivid and marketable one. Specifically, to remove the bluish hue in purple sapphires and rubies to end up with a vivid (‘hot’) pink or pure red colour, or to introduce an orange hue in pink sapphires, resulting in the best case in a pinkish orange colour as known for Padparadscha sapphire.

Figure 2: LINAC gamma irradiation source, a gyroscopic radiosurgery platform, installed at SNRC, Zurich, Switzerland.

Irradiation experiments carried out by SSEF

Since 2022, the Swiss Gemmological Institute SSEF has been collaborating with the Swiss Neuro Radiosurgery Centre SNRC (Zurich) to better understand the effects of gamma irradiation on chromium-bearing corundum (rubies and pink sapphires) and to develop a testing method for detecting such irradiation treatment. In a series of experiments, corundum samples ranging from light pink (from Madagascar) to dark red colour (from Mozambique) were treated by gamma radiation using a LINAC instrument (ZAP-X, gyroscopic radiosurgery platform, USA) (Figure 2). This device features a well-defined 3D radiation profile and offers accurate radiation dose control. The samples were exposed to a total radiation dose of 10,000 Gray in increments of 2,000 Gray. Some of the samples were heated (1200°C for 10h) before gamma irradiation to eliminate any potential residual effects from previous treatments on the host gemstone and structurally ‘reset’ (anneal) any present zircon inclusion. The reason for this pre-treatment heating was to make the possible colour shift by gamma irradiation more pronounced and discernible. Following the irradiation experiments, immediate visual inspection was conducted. Various analytical methods, such as UV-VIS absorption spectroscopy of the corundum and Raman analysis of the zircon inclusions in pink sapphires were conducted to compare the results before and after gamma irradiation. To determine the colour stability of the irradiated gemstones, a standard stability test as well as a final annealing step (500°C) were applied.

In a second experiment run-up, we recently submitted a number of purplish sapphires from Madagascar anonymously to a facility for a so-called ‘hospital’ irradiation treatment (see Figure 1). These samples were kindly donated to SSEF by a supportive member of the trade. Again, all these samples were meticulously analysed at SSEF before and after irradiation. They are currently being inspected using further advanced analytical methods in collaboration with Swiss academic research institutions to fully document any changes which occurred by this treatment. This 2nd experiment is still an ongoing research study, which means that more detailed results of this experiment will only be presented in the near future.

Effect of irradiation on corundum: results of our experiments

Our experiments using the LINAC instrument reveals that gamma irradiation (at least with our chosen dose and experimental conditions) has no discernible effect on the colour of ruby of strong red saturation (from Mozambique). This is in line with results by other international research groups and labs. It may have, however, a certain influence on rubies of purplish hue and lower colour saturation as described by Suwanmanee et al. (2023), shifting their colour to a more attractive red hue.

Figure 3: Visual inspection of ruby (six dark red slices, cut from a single stone) initially and after gamma radiation treatment with incremental change in radiation dose. No colour shift is observed. Photo: SSEF.
Among our specimens, sample #3, a light pink sapphire, is the only one which exhibits a (quasi) stable colour shift to orange after gamma irradiation, with only a slight decrease in orange saturation after four months— thus remarkably not reverting to its initial light pink colour. This stability persisted to some extent even after a colour stability test with the sample retaining a weaker but still orange hue until it was subjected to an annealing step, which finally restored its original light pink colour (Figure 5). This exceptional case suggests the presence of at least two distinct types of orange colour centres within these samples: one unstable and one stable. The unstable colour centre likely contributes to the colour reversion observed in the other samples, while the stable one accounts for the prolonged retention of the orange hue post-gamma irradiation. Sample #3’s behaviour provides valuable insights into the mechanisms of colour shift in pink sapphires, indicating a complex interplay between different colour centres and the potential for targeted colour modifications through this specific treatment.
Figure 5: Colour shift of Sample #3, a light pink sapphire, after each treatment step. From left to right, different treatments show the colour of the stone shifting from light pink to intense orange and reduce to light orange, and finally back to the original light pink colour. The colour was analysed by Gem Colour Analyzer at SSEF after each experiment step. Trace element concentrations of the stone are provided to the right. Figure: H.A.O. Wang, SSEF.
In summary, these findings suggest a differential response to gamma irradiation across corundum varieties, potentially influenced by their chromium (Cr) content (Powell 1966). The varying effects of gamma irradiation might be attributed to the difference in Cr concentration, with high-Cr corundum (ruby of saturated red colour) exhibiting resilience to colour modification compared to low-Cr corundum (pink sapphire). This distinction raises intriguing questions about the structural and compositional factors contributing to the observed behaviours, pointing towards the need for further research to understand the mechanisms underpinning the differential sensitivity to gamma irradiation within corundum varieties. In search of a detection method for this treatment UV-VIS spectroscopy is a powerful technique that can capture the detailed absorption behaviour of colour centres in a gemstone. Our analysis of Sample #3, depicted in Figure 6, showcases the spectral changes following each experimental step. Notably, there is an observed enhancement in absorption at approximately 320nm and 475nm post- gamma irradiation (green trace in the spectrum). Unlike the effects of heat treatment at 1200°C which did not alter theabsorption at 475nm, gamma irradiation distinctly increased absorption at this wavelength. This phenomenon is likely attributable to the formation of trapped holes (O-1) adjacent to Fe3+ or Cr3+, substituting for Al3+ in the crystal structure of corundum, thereby generating two types of colour centres responsible for the shift of the initially light pink gemstone to an intense orange colour. This mechanism, facilitating the creation of trapped holes and consequent colour alterations in Cr- and Fe-containing corundum, aligns with previous observations on natural untreated corundum of orange colour (Dubinsky et al., 2020). Moreover, our analyses reveal that a subsequent colour stability test (using a strong daylight source) does not necessarily revert the spectrum to its original state, leaving in our sample #3 a persistent light orange hue. Besides investigating the before/after irradiation properties of our corundum samples (aka host gems), we also looked whether inclusion features in a corundum may reveal evidence of a gamma irradiation treatment. We specifically focused our analyses on the lattice disorder of zircon inclusions within one of the pink sapphires from Ilakaka, Madagascar (light pink Sample #3) before/after irradiation. This investigation was inspired by literature indicating that Raman peak broadening, a proxy for increased lattice disorder (Nasdala et al., 1995), could result from gamma radiation (Zhu et al., 2015). We focused on the behaviour of two characteristic Raman peaks at 974 cm-1 and 1008 cm-1. Our experimental results, derived from five zircon inclusions, reveal no statistically significant broadening of the Raman peaks following gamma irradiation (Figure 7). To kind of ‘reset’ the structural disorder of these zircons, the sample was heated prior to the irradiation treatment. As detailed in Figure 7, the full width half maximum (FWHM) values for both peaks remained consistent with those observed in the ‘reset state’ after heat treatment, indicating no notable increase in lattice disorder by the irradiation experiment. This observation was further supported by the analysis of the mean FWHM values across the inclusions and the variability of these values, as measured by standard deviation (STD) and relative standard deviation (RSD).
Figure 6: UV-VIS spectra comparison of Sample #3, a light pink sapphire, going through a series of treatments. Note the visible colour range bar on the top, which indicates that only spectral changes between 380nm and 700nm may be noticed by our eyes. Figure: H.A.O. Wang, SSEF.

Interestingly, while gamma irradiation altered the colour of the light pink sapphire significantly, it did not affect the structural integrity of the zircon inclusions to a measurable extent. In contrast to this, the heat treatment applied prior to gamma irradiation markedly decreased the FWHMs and RSDs of the zircon Raman peaks, suggesting a reduction in lattice disorder. These findings suggest that the dose of gamma radiation used in our study was insufficient to induce detectable changes in the zircon inclusions, contrary to expectations based on previous literature. Consequently, analysing zircon crystal disorder post-gamma irradiation may not currently serve as an effective method for detecting this irradiation treatment.

Figure 7: Raman analysis of zircon inclusions in Sample #3 (light pink sapphire), which retains a light orange colour after gamma irradiation and a colour stability test. Top: morphology of zircon inclusions at their initial state, after 1200 ̊C heat treatment, and after gamma irradiation. Bottom: Statistical summary of Raman spectroscopy peaks (at 974 cm-1 and 1008 cm-1) including full width half maximum (FWHM), standard deviation (STD), and relative standard deviation (RSD) for each peak. FWHM serves as a measure of crystal disorder, with higher values indicating increased disorder. RSD values assess the variability of FWHM across the inclusions, with higher RSDs suggesting greater differences in crystal disorder levels among the inclusions.

Outlook: what is to come?

Based on our current research results and after discussion with other gem labs, it has to be said that to this day, there is no criterion known which allows an unambiguous detection of gamma irradiation treated corundum.

Ongoing research at SSEF is focused among other things on exploring the relationship between the Cr-related photoluminescence (R-line intensity) and Cr-concentration, all with the aim to find a possible detection method for gamma-irradiated ruby and pink sapphires. It is essential to note that the detection of this irradiation treatment will likely require not only one criterion and detection method, but a combination of tests and methods, following the complexity of the effects of this treatment on corundum of different trace element composition and from different geological and geographical settings. This is even more true, as natural untreated corundum may have experienced natural irradiation if exposed to fluids containing radioactive elements or by natural irradiation in the host rock (Nassau and Valente 1987).

It currently is only possible to detect the treatment when the stone has also been analysed in its pre-treatment state and is compared and again analysed after irradiation. We know that this is a challenging situation for the trade and labs alike and would like to emphasize that we do the utmost in research trying to find solutions for the trade against this challenge and will update the trade regularly about any new findings in this matter.

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