Chromophore Behaviors (Including the 880 nm Absorption Band) in an Irradiated Pink Sapphire

Wasura Soonthorntantikul, Wim Vertriest, and Aaron Palke

Irradiation is a known process for enhancing color in corundum, particularly yellow and pink sapphire. Recently, some rubies and pink sapphires with bluish tint were reportedly treated with radiation (“LMHC makes progress on laboratory report harmonisation, discussed current challenges in detection of corundum treatments,” LMHC press release, March 14, 2023). However, chromophore behaviors in irradiated ruby/pink sapphire have not been fully investigated. This report shows preliminary results of an experiment involving irradiated pink sapphire. An untreated Madagascar pink sapphire with a blue modifier (figure 1) was fabricated as an optical wafer with two polished faces perpendicular to the c-axis for ultraviolet/visible/near-infrared (UV-Vis-NIR) spectroscopic study. Trace element chemistry was analyzed in the UV-Vis-NIR measurement area using laser ablation−inductively coupled plasma−mass spectrometry. This information can be used to link the visual color and the color-causing trace elements listed in figure 1.

The spectrum of the untreated sample displayed broad absorption bands at ~400 and ~560 nm together with weak peaks at ~470 and at 693 nm, demonstrating that the Cr³⁺ chromophore was responsible for the pink color (blue line in figure 2; see E.V. Dubinsky et al., “A quantitative description of the causes of color in corundum,” Spring 2020 G&G, pp. 2–28). This sample also showed a weak broad band at around 580 nm related to Fe²⁺-Ti⁴⁺ pairs, which produced a blue color, modifying the pink color from Cr³⁺ to purple-pink. Weak Fe³⁺-related absorption features were also observed, represented by a broad band at ~330 nm and narrow peaks at 377, 388, and 450 nm. The iron-related chromophore only causes significant yellow color at relatively high iron concentrations (greater than ~1000 ppma) due to Fe³⁺ chromophores’ weak color strength.

For our radiation experiment, the sample was treated with a gamma irradiation dose of 500 kGy at the Irradiation Center at the Thailand Institute of Nuclear Technology (public organization). After irradiation, the pink sapphire changed to a padparadscha-like coloration in which the subtle blue color was partially removed (figure 1). The UV-Vis-NIR spectrum after irradiation showed a significant increase in absorption intensities at wavelengths shorter than ~560 nm or longer than ~660 nm (red line in figure 2). To observe changes in chromophore behavior caused by irradiation, the spectrum after irradiation was subtracted from the pretreatment spectrum. The difference spectrum (green line in figure 2) revealed that this acceptor-dominated corundum with an atomic concentration of magnesium greater than the sum of titanium and silicon concentrations created a strong absorption feature below ~560 nm to the UV region, which matches well with the trapped hole associated with the Cr³⁺ spectrum for causing orange coloration (Dubinsky et al., 2020). Using the absorption cross section information provided in Dubinsky et al. (2020), the concentration of trapped hole-Cr³⁺ chromophores induced by irradiation was approximately 0.8 ppma in the sample. This chromophore had very high color strength, and therefore a noticeable color change could be observed with only a tiny concentration of the trapped hole paired with Cr³⁺ created after treatment. The Fe²⁺-Ti⁴⁺ chromophore also reduced by approximately 0.7 ppma in the treated sample, resulting in less blue color modification. This slight lightening in blue color after irradiation is possibly due to the change in oxidation state of iron (which would reduce the number of Fe²⁺-Ti⁴⁺ pairs). In addition, the creation of orange color (trapped hole-Cr³⁺) masked a certain portion of remaining blue color after treatment, as blue and orange are complementary colors. The change in UV-Vis-NIR spectrum corresponded with the change in color appearance caused by irradiation.

When studying absorption features in the UV-Vis-NIR spectrum of corundum, a broad band centered at around 880 nm in the red to near-infrared region is typically associated with basalt-related blue sapphire (e.g., A.C. Palke et al., “Geographic origin determination of blue sapphire,” Winter 2019 G&G, pp. 536–579). This band has little or no impact on color. In addition to basalt-related blue sapphires that are exposed to heat naturally, the 880 nm band can be induced in the spectrum of a metamorphic-type sapphire that has been heat treated, such as heated Madagascar sapphire (E.B. Hughes and R. Perkins, “Madagascar sapphire: Low-temperature heat treatment experiments,” Summer 2019 G&G, pp. 184–197) or heated Rock Creek sapphire (J.L. Emmett and T.R. Douthit, “Heat treating the sapphires of Rock Creek, Montana,” Winter 1993 G&G, pp. 250–272). This feature can also be found in Mozambican ruby after heat treatment (S. Saeseaw et al., “Update on “low-temperature” heat treatment of Mozambican ruby: A focus on inclusions and FTIR spectroscopy,” GIA Research News, April 30, 2018). The origin of this band is not fully understood but may be related to iron clusters.

The 880 nm band was absent in the UV-Vis-NIR spectrum of this sample before treatment. Interestingly, this pink sapphire developed the 880 nm band after irradiation, with a significant intensity around 1.2 cm^–1. Color stability testing is usually performed in gem laboratories for yellow to padparadscha sapphires that may contain unstable color centers (e.g., A.C. Palke et al., “An update on sapphires with unstable color,” GIA Research News, December 12, 2022; Summer 2022 GNI, pp. 259–260). After the color stability test (figures 1 and 3), the orange coloration introduced by irradiation faded away after exposure to intense incandescent light due to the dissociation of the trapped hole paired with Cr³⁺, whereas the 880 nm band remained. This preliminary observation suggests that the 880 nm band can be produced by an irradiation process, and this feature could provide some interesting evidence of treatment, either heat or irradiation, in pink sapphires and rubies. However, further studies would be needed to understand the development of the 880 nm absorption band.