Natural unheated spinel

Figure 1 shows the characteristic photoluminescence reaction of chromium in natural spinel crystal lattice. Strong emission peaks at red (starting from 630nm) are known by gemologists as 'organ pipes'. Almost all spinel colors contain sufficient amount of Cr³⁺ for showing this luminescence reaction. If one looks at the spectra more carefully it is clear that this reaction is very consistent in peak positions and their relative intensity for varying spinel colors. This spectrum is so easy to memorize that usually gemologist knows it's a natural spinel long before the Raman spectrometer has accomplished the library comparison.

 Figure 1: PL (GemmoRaman-532nm) spectra of Natural unheated spinel (#117 and #136 are shifted vertically for clarity)

Flame fusion synthetic spinel

If well ordered spinel crystal lattice receives heat above 800°C it starts to disorder. Disordering shows up in the PL spectrum so that emission peaks start to broaden, merge and shift. In other words, the spectra in Fig1. proves both the natural origin and the fact the stone has not received high temperature heat treatment for improving it's color or clarity.

Synthetic spinel manufactured by Verneuil method has been "heavily heat treated" during the flame fusion process, so the crystal lattice is never in very good order. While color variations and chromophores used in manufacturing of these synthetics are endless, they almost always have high enough chromium content for triggering the PL reaction. This fact applies even for colorless variation. Disorder of the crystal lattice broadens the main peak (the so called zero phonon line) significantly and also shifts it's position about 4-5 nm towards IR. Minor peaks have been broadened and merged. Please note this kind of spectrum is NOT indicative for Verneuil method as it is possible to obtain similar results from natural heat treated spinel as well. It is mandatory to continue tests with refractometer and microscope for making the final decision. It is also worth noting that chrome bearing tourmaline has a very similar PL spectrum. It is a good idea to perform Raman fingerprint analysis to make sure one is looking at spinel instead of tourmaline.

Figure 2: PL (GemmoRaman-532nm) spectra of synthetic spinel, flame fusion method (#142, #127 and #129 are shifted vertically for clarity)

Heat treated natural and flux synthetic spinels

Flux melt process temperatures creates disorder in a spinel's crystal structure as well. These stones can be extremely difficult to distinguish from natural spinel by traditional methods, so here PL spectrometer provides an fast and easy solution. Two samples in Fig 3. are red (Cr) and blue (Co, Cr) flux synthetic spinels manufactured in Russia.

Figure 3: PL (GemmoRaman-532nm) spectra of synthetic spinel, flux melt method (#126 is shifted vertically for clarity)

Interpretation of spinel Raman peaks

If spinel does not contain enough chromium to make the difference, one can focus on acquisition of much weaker Raman fingerprint peaks of spinel instead of PL reaction. These peaks broaden and red-shift as a consequence of heat induced deformation of the crystal lattice. Additionally, non-stoichiometric chemical composition (excess amount of aluminum) in Verneuil spinel introduces new peaks to the Raman spectrum. Distinguishing natural unheated inclusion free spinel from synthetic spinel is simply a matter of looking at the line width and position of the 405 cm^-1 Raman peak.

Figure 4: Raman fingerprint (GemmoRaman-532nm) spectra of natural unheated spinel vs. flame fusion synthetic spinel