2024

Low-temperature heating of corundum: possibilities and challenges for detection

By Dr. M.S. Krzemnicki, first published in Facette 29 (May 2024)

Heat treatment is generally applied to gemstones to change and enhance their colour. Over the past few decades, it has become a very common practice for certain gem species, such as aquamarine, amethyst (resulting in citrine), copper-bearing tourmalines (Paraiba), tanzanite, and notably for any colour variety of corundum. (Figure 1). But it is important to know that heat treatment of gems has been known since historic times and was described and practiced many centuries ago (see Hughes et al. 2017, and Notari et al. 2019, and references therein). A historic account describing the effect of heating rubies is found in the Book on Mineralogy (trans. Said, 1989) by Al-Biruni (AD 973–1048):

The practice is therefore, to roast the reddish [stones] so that the mixed colours are made to disappear…A ruby stone having been roasted is re- examined, and, in case it does not gain clarity, it is re-heated. ….

This early account explains perfectly the effect of heating on rubies with a purple/blue zone (see Figure 3), and also how even today, a heat treatment can be repeated if the first attempt was unsuccessful.

The effect of heat treatment on a gemstone depends on several different factors. These include external parameters, such as the duration of treatment, gradient of heating and/or cooling, atmosphere (e.g. oxidizing or reducing), and most importantly the peak temperature reached during such a treatment. Intrinsic properties of the gemstone such as the presence and distribution of colouring elements and their valency- state and aggregation and the presence of lattice defects acting as colour centres define how the gemstone responds to a heating process.

Figure 1: Unheated (left) and heated (right) gemstone pairs indicating the colour change intended by a heat treatment. Starting material in the upper row is greenish blue aquamarine (left), amethyst (middle) and purple Cu-bearing tourmaline. Starting material in the lower row is greyish ‘geuda’ sapphire, purple sapphire and purplish ruby. The collage of these gemstones has been digitally arranged. Photo: M.S. Krzemnicki, SSEF.
Figure 2: Approximate temperature ranges for the heat treatment of selected gemstones (based on gemmological literature). Figure: M.S. Krzemnicki, SSEF.
When a gemstone is heated, the result may range from no visual change to a distinct change/enhancement of colour, but also to cracking, and in the worst case (overheating) to partial degradation, a complete structural phase transformation, or even melting. Over the last centuries, more specific heat treatment processes have been developed, based mostly on the same principle (heating in air) but using different temperatures adapted to each gem species (Figure 2). Most gem materials are heated at temperatures well below 1000 °C, whereas heating of corundum that has a visible effect on colour has generally been documented only at rather high temperatures.
Figure 3: Unheated Mozambique ruby of very fine quality showing a diffuse bluish zone in a corner. It may be possible to remove such a bluish zone by a low-temperature heat treatment. However, the trade nowadays prefers to keep these rubies unheated, if such a bluish zone is not affecting the beauty of the stone. The reason is that the price of unheated stones is much higher than of that of heated stones. Photo: L. Phan, SSEF.

Heat treatment detection of corundum

For corundum -Al2O3 (ruby and sapphire and other colour varieties)- various heat treatment processes with or without additives have been developed, specifically in the last few decades (Figure 4). They are widely applied in the gem trade (e.g. in cutting/manufacturing and trading hubs in Sri Lanka, Thailand, India) to enhance the colour and additionally improve -in some cases- the transparency and stability of certain types of corundum. With such heat treatments, it is possible to modify rubies (and other varieties of corundum) of lower quality into visually more beautiful stones, and consequently guarantee a steady supply of gems to the international market.

Figure 4: Selected treatment options for corundum. For heat treatment, a range of options are possible, either without or with additives. Figure: M.S. Krzemnicki, SSEF.

Heat treatment of ruby and other corundum varieties is usually carried out at temperatures ranging from 700 to 1800 °C. Historically, this has been done using a simple and artisanal blow-pipe (e.g. in Sri Lanka) reaching temperatures of about 1200 °C. Heating has evolved into a multitude of treatment options, today often carried out under controlled conditions (atmosphere and pressure) in electric muffle furnaces (Nassau 1981; Themelis 1992; GIT 2019). Heating of corundum at higher temperatures (> 1200 °C) may have a considerable impact on its internal features and colour, notably local discoid expansion cracks may develop around solid and fluid inclusions and tiny rutile (TiO2) needles commonly present in corundum as so-called ‘silk’ may (partially) dissolve. In contrast to this, a heat treatment process in the range of about 700 – 1000 °C, known in the trade as ‘low-temperature’ heating, usually results only in a slight (but desirable) shift of colour and may leave inclusions basically unaltered.

Traditionally, the detection of heat treatment in corundum is mainly based on meticulous microscopic observation. Because of the heating process, internal features may be affected and altered as mentioned above and thus provide a clear indications of heat treatment. However, in corundum which is heated at so-called low temperatures (about 700 °C to 1200 °C) only very minute or even no alterations of inclusions may be observed under the microscope (Figure 5).

Figure 5: The lower the heating temperature, the less evident are microscopic features indicating such a heat treatment. From distinct discoid expansion features (left), through tiny and very tiny discoid features and spotty appearance of platelets in rubies heated at so-called low temperature. Microphotos: M.S. Krzemnicki, SSEF.

Therefore, the detection of heat treatment in corundum today mostly relies on a combination of infrared spectroscopy (FTIR) and Raman micro-spectroscopy, in addition to ‘classic’ microscopic observations.

In FTIR analyses of corundum, the focus lies very much on the presence and intensity of OH- related absorption peaks. Namely the 3309, 3232 and 3185 cm-1 series in metamorphic corundum is considered a strong indication for artificial heat treatment (Smith 1995; Beran & Rossman 2006; Balmer et al. 2006; Pardieu et al. 2015; Krzemnicki 2019; Saeseaw et al. 2020). The presence or absence of a Mg-O related band at 3160 cm-1 or broad boehmite and diaspore bands in the 3500-2500 cm-1 range are other important criteria, as these bands are reduced or completely disappearing during heating (Smith & van der Bogert 2006). Another approach is to determine the peak width (FWHM) of the main Raman peak of zircon inclusions. This approach has been explored more recently (Wang et al. 2006; Krzemnicki et al. 2021; Karampelas et al. 2023), notably in pink sapphires from Ilakaka (Madagascar), as a way to detect heat treatment.

However, both FTIR and Raman have certain limitations, i.e. hydroxide- related bands in infrared spectra are not always present in heated rubies and sapphires (Saeseaw et al. 2018, Pardieu et al. 2015) and the FWHM of the main Raman peak of zircon inclusions in unheated and heated corundum may show considerable overlap (Krzemnicki et al. 2021, Karampelas et al. 2023). Additionally, this Raman bandwidth in zircon inclusions is strongly dependent on the concentration of radioactive trace elements, crystallinity (metamictisation), and finally geological and geographic origin where the corundum has formed (Nasdala et al. 1995 and 2001; Xu & Krzemnicki 2021).

Recently, we published a new scientific Raman study using the presence of the oxyhydroxides diaspore and goethite as inclusions in corundum to separate unheated corundum (and other gems) from heated stones (Figure 6). The study, entitled Dehydration of Diaspore and Goethite during Low-Temperature Heating as Criterion to Separate Unheated from Heated Rubies and Sapphires, is freely accessible on the internet (https://doi.org/10.3390/min13121557) or from the research library on our SSEF website. For this study, we carried out heating experiments on selected corundum samples containing epigenetic diaspore α -AlO(OH) and goethite α-FeO(OH) inclusions, to document the phase transformation and dehydration of these hydroxides into the anhydrous oxides corundum α-Al2O3 and hematite α-Fe2O3 upon heating. Both phase transformation systems have already been studied extensively in the past decades (Ervin 1952; Lima-de-Faria 1963; Frost et al. 1999; Majzlan et al. 2003; Gialanella et al. 2010; Koivula 2013; Sripoonjan et al. 2016). Both hydroxides, diaspore and goethite are known to be thermally stable only up to about 350 °C (goethite) and 500 °C (diaspore) and to transform to their respective Al- and Fe-oxide phases when further heated.

During our heating experiments, selected diaspore and goethite inclusions in corundum samples were monitored using a Raman microprobe and their phase transformation and dehydration to corundum and hematite could be clearly documented in all samples (Figure 7).

Figure 6: Title page of our most recent scientific publication about heat treatment detection of corundum. Source: Minerals, MDPI.
Figure 7: Raman spectra of the same diaspore inclusion taken after stepwise heating from room temperature to 700 °C. In a narrow temperature range between 525 and 550 °C, the diaspore dehydrates and transforms to corundum. Figure: M.S. Krzemnicki, SSEF.
Interestingly, this dehydration occurred for both inclusion minerals quite abruptly in a narrow temperature range (525 to 550 °C for diaspore to corundum; 300 to 325 °C for goethite to hematite). These ranges are far below heating temperatures commonly applied on corundum, but specifically for goethite even lower than those applied on many other gemstones (e.g. Paraiba tourmaline). Therefore, it is valid to say that Raman spectroscopy has proven to be a very useful analytical method to document these phase transformations and can thus be considered a very promising tool to separate unheated from heated gem-quality ruby and other corundum varieties and even other gemstones. In the past few months, we have applied these phase transformation markers successfully on numerous client stones. By adopting this approach, we were able to conclude a treatment status even in cases in which microscopic features or FTIR analyses were non-conclusive.

References

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