Gem News International Gems & Gemology, Spring 2024, Vol. 60, No. 1

Identification of Chromium-Bearing Red Gemstones Using Photoluminescence: A Red Musgravite Case Study

Taku Okada and Makoto Miura

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Figure 1. A 1.59 ct red musgravite. Photo by Shunsuke Nagai.
 
Figure 1. A 1.59 ct red musgravite. Photo by Shunsuke Nagai.
Table 1. Trace-element concentrations (in ppmw) of the red musgravite sample, measured by LA-ICP-MS.
 
Figure 2. The PL spectrum of the red musgravite consisted of two very strong peaks at 685.7 nm (R<sub>1</sub>) and 686.8 nm (R<sub>2</sub> or L). In the spectrum below that, its weak side peaks are expanded by multiplying the intensity by 20. They are partially similar to those of ruby (692.9 (R<sub>2</sub>) and 694.4 (R<sub>1</sub>) nm), its expanded weak side bands, and a red spinel (685.5 (R) and 687.1 (L) nm). Spectra are offset vertically for clarity.
 
Figure 2. The PL spectrum of the red musgravite consisted of two very strong peaks at 685.7 nm (R1) and 686.8 nm (R2 or L). In the spectrum below that, its weak side peaks are expanded by multiplying the intensity by 20. They are partially similar to those of ruby (692.9 (R2) and 694.4 (R1) nm), its expanded weak side bands, and a red spinel (685.5 (R) and 687.1 (L) nm). Spectra are offset vertically for clarity.

Photoluminescence (PL) provides us with useful information to understand physical properties of gem materials and can also be used for gem identification and treatment detection. For example, a 1.59 ct red musgravite (figure 1) that had previously been examined by the Central Gem Laboratory in Japan (Z. Zhenghao et al., “A remarkable Cr-bearing red musgravite,” Journal of Gemmology, Vol. 38, No. 6, 2023, pp. 548–551) was also submitted to GIA’s Tokyo laboratory for identification. Its standard gemological properties (refractive index, specific gravity, dichroism, and UV fluorescence reaction), Raman spectrum, and ultraviolet/visible spectrum were consistent with musgravite. The chemical formula was measured as Be1.21(Mg1.95, Fe0.01, Zn0.02)Al5.86O12 by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) with a chromium concentration of 1360–1840 ppm (table 1). A PL spectrum for musgravite (figure 2) was also collected with a 532 nm laser and consisted of two strong peaks and weak side bands. Interestingly, the positions of the strong peaks were close to those caused by chromium in spinel, but the overall pattern was similar to that caused by chromium in ruby or sapphire. PL emission spectra are collected by exposing a material to strong short wavelength light, creating excited energy states in the material. These excited energy states then relax and return to a ground state by emitting light at a longer wavelength than the excitation wavelengths. The light collected during this relaxation is the PL spectrum. Trace amounts of chromium in many materials can easily enter these excited energy states and produce characteristic PL spectra in different gems such as ruby, spinel, and musgravite.

Figure 3. Polyhedral representations of the crystal structures of (A) reported musgravite, BeMg<sub>1.63</sub>Fe<sub>0.37</sub>Al<sub>6</sub>O<sub>12</sub> (Nuber and Schmetzer, 1983); (B) typical corundum, Al<sub>2</sub>O<sub>3</sub>; and (C) normal spinel, MgAl<sub>2</sub>O<sub>4</sub>. Thin black lines depict the unit cells. The green tetrahedra are BeO<sub>4</sub>. The orange tetrahedra and octahedra are MgO<sub>4</sub> and MgO<sub>6</sub>, respectively. The blue tetrahedra and octahedra are AlO<sub>4</sub><sub> </sub>and AlO<sub>6</sub>, respectively. In musgravite, the AlO<sub>6</sub> octahedra have three types: one undistorted (Al1) and two differently distorted (Al2 and Al3).
 
Figure 3. Polyhedral representations of the crystal structures of (A) reported musgravite, BeMg1.63Fe0.37Al6O12 (Nuber and Schmetzer, 1983); (B) typical corundum, Al2O3; and (C) normal spinel, MgAl2O4. Thin black lines depict the unit cells. The green tetrahedra are BeO4. The orange tetrahedra and octahedra are MgO4 and MgO6, respectively. The blue tetrahedra and octahedra are AlO4 and AlO6, respectively. In musgravite, the AlO6 octahedra have three types: one undistorted (Al1) and two differently distorted (Al2 and Al3).

In this report, we investigated the reasons why these PL spectra differ between red musgravite, ruby, and spinel. The two strong peaks at 685.7 and 686.8 nm of the red musgravite are referred to as zero-phonon lines (ZPLs) that do not contain the energy of “lattice vibrations,” which are the motions of atoms around their equilibrium positions. These peaks result from the transition from the lowest excited state to the ground state of three 3d orbital electrons of the Cr3+ ion replacing the Al3+ ion at the center of oxygen polyhedra, as in ruby and red spinel. The crystal structure of musgravite includes one type of AlO4 tetrahedron and three types of AlO6 octahedron: one undistorted and two differently distorted; see figure 3A, drawn using VESTA software (K. Momma and F. Izumi, “VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data,” Journal of Applied Crystallography, Vol. 44, 2011, pp. 1272–1276) and the reported crystal structure data (B. Nuber and K. Schmetzer, “Crystal structure of ternary Be-Mg-Al oxides: taaffeite, BeMg3Al8O16, and musgravite, BeMg2Al6O12,” Neues Jahrbuch fur Mineralogie, Monatshefte, 1983, pp. 393–402). The chromium PL emission is split into two strong lines, due to either the distortion of the AlO6 octahedron as in ruby and sapphire, or the occupation of multiple crystal sites by chromium. The position of the red musgravite is shorter in wavelength than in ruby and sapphire but almost equal to that of red spinel. This means the energy difference between the excited and ground states is greater than that in ruby and close to that in red spinel.

The chromium PL spectrum for musgravite shows additional small peaks at longer and shorter wavelengths, termed “phonon sidebands” and “Urbach tails,” respectively (again, see figure 2). If the intensity of the main ZPLs is a large proportion of the total luminescence intensity, it indicates that the Debye-Waller factor, representing the magnitude of atomic vibrations in a crystal structure, is small. The weak sidebands on both sides of the red musgravite indicate that the Debye-Waller factor is smaller than that of red spinel but similar to ruby.

PL spectroscopy on chromium-bearing gemstones is not only one of the most powerful tools for quick and nondestructive identification but also a means to obtain interesting physical properties of rare gemstones by comparing with the PL spectra of familiar gems. Furthermore, the difference in degree of the Debye-Waller factor may be one of the reasons why the presence or absence of heat treatment can be detected by PL in spinel (S. Saeseaw et al., “Distinguishing heated spinels from unheated natural spinels and from synthetic spinels: A short review of on-going research,” https://www.gia.edu/doc/distinguishing-heated-spinels-from-unheated-natural-spinels.pdf, April 2, 2009), but not in corundum.

Taku Okada is a staff gemologist, and Makoto Miura is supervisor of colored stone identification, at GIA in Tokyo.