FTIR in Gem Testing • A Pink Sapphire Lesson

by Richard Hughes

A discussion of DRIFTS vs the Beam Condenser FTIR attachment in the testing of pink sapphire.

FTIR in Gem Testing • A Pink Sapphire Lesson

In January 2020, a pair of earrings were brought to Lotus Gemology's Bangkok lab for testing. We were asked to test the pink sapphire at the center of each earring.

The first gem tested showed inclusion evidence suggesting it had been subjected to heat treatment. But in the second gem, the inclusion evidence was inconclusive and partially obscured by the mounting.

Under the normal testing protocol at Lotus Gemology, all corundums will have their infrared spectra measured using a Bruker Tensor 27 FTIR unit with a Pike EasyDiff™ diffused reflectance (DRIFTS) collection device.

Due to the mounting, we were forced to use a Pike 4x beam condenser attachment to measure the infrared spectrum. The result was the spectrum shown in Figure 1.

A 003 5750 BC 64Figure 1. Infrared spectrum of a Madagascar pink sapphire using the Pike 4x beam condenser collection attachment. This sends a narrow beam through the gem. Using this technique, the 3309 cm-1 was at the noise floor, while the 3232 cm-1 peak was not visible at all.

Because the inclusion and spectral evidence was inconclusive regarding heat treatment, we requested that the client unmount the stone. After unmounting, we again measured the infrared spectrum, this time using the DRIFTS attachment. The resulting spectrum is shown in Figure 2, where both the 3309 cm-1 and key peak at 3232 cm-1 rise above the noise floor. This clearly demonstrates why DRIFTS is the better choice for measuring the infrared spectra of most gems.

A 003 5750 DR 64Figure 2. Infrared spectrum of the same Madagascar pink sapphire using the DRIFTS (diffuse reflectance) attachment. This bounces the beam through the gem from a variety of directions. Using this technique, the 3309 cm-1 rises well above the noise floor. In addition, the peak at 3232 cm-1 rises above the noise floor, proving that the gem was subjected to artificial heat treatment.

Discussion

Over the years, gemologists have measured the infrared spectra of tens of thousands of rubies and sapphires. The appearance of a peak at 3232 cm-1 has proven to be an extremely strong indicator of heat treatment (Atichat et al., 2011). Crystals by definition display directional properties, and their spectra are no exception. Because the DRIFTS attachment samples light moving through the gem in a variety of directions, it is more likely to reveal information such as the key 3232 cm-1 peak (Hughes et al., 2007). Thus, it should be the first choice for gemologists measuring infrared spectra.

Conclusion

The FTIR spectrum has proven to be an essential technique in unmasking heat treatment in both ruby and sapphire. But like any tool, it requires a skilled operator to extract its full potential. Part of that skill is the choice of the collection device (DRIFTS vs. beam condensor). In addition, it is crucial that clients be warned about the perils of testing gems while mounted. Mountings severely restrict the types of tests that can be performed, as well as the information gathered by those tests. All gems tested by Lotus Gemology in the mounted state feature the following warning on the report: "Mounted gems are tested only to the degree permitted by the mounting. Results may change following unmounting."

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References

  • Atichat, W., Leelawatanasuk, T., Sriprasert, B., Pisutha-Arnond, V., Wathanakul, P. and Suthirat, C. (2011) Mozambique ruby: Indication of low-temperature heat treatment. 32nd International Gemmological Conference, Interlaken, Switzerland, pp. 157–159; RWHL.

  • Hughes, R.W., Castoro, L., Nyunt, H-P. K. & Kiefert, L. (2007) FTIR in Gem Testing • FTIR Intrigue. LotusGemology.com; accessed 13 Feb. 2020.

 

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