by Frank J. Yang, Gerard Pereira, and Liang Don Chen
AL
Using Raman Spectroscopy for Gemstone Analysis
Traditional gemstone analysis methods in- clude the scratch test, UV light test, fog test, conductivity test, loupe test, transparency test, heat test, and density test. The scratch test and the heat test must be done carefully, as they can damage gemstones; nondestructive analysis can be done using the conductivity test and optical methods such as the UV light test and refractive index test, and methods that detect color changes between natural light, incandescent light, and fluorescent light. Refractive index (RI) testing is useful for dis- tinguishing between natural and synthetic sapphires. A refractometer can measure the RI of a transparent stone to determine the degree of light-path divergence within an optically transparent stone. Each optically transparent gemstone has its own RI value, which indicates quality. Ultraviolet-induced fluorescence radiation is used in many labo- ratories to identify synthetic stones, as they exhibit abnormal fluorescence. The exception is the synthetic orange sapphire, as the natural orange sapphire is itself highly fluorescent and not easily distinguishable from the synthetic product. Optical test methods may be suit- able only for optically transparent or partially transparent gemstones, and secondary ion mass spectrometry (SIMS) is expensive, labor- intensive, and requires expertise. In addition, the above methods do not identify the chemi- cal nature and origin of gemstones.
As an alternative to the above testing meth- ods, Raman spectroscopy is nondestructive, extremely fast, and low in cost. It is also sensi- tive to highly symmetric covalent bonds that possess little or no natural dipole movement. The Raman fingerprint allows gemologists to identify the chemical identity and origin of the
Figure 1 – Comparison of spectra in terms of Raman shift for diamond, silicon, and germanium (© Copyright Thermo Fisher Scientific , Waltham, MA. All rights reserved. Image reproduced with per- mission of Thermo Fisher Scientific.)
Figure 2 – Comparison of Raman spectra of a natural type A jade, resin-filled type B jade, and quartz. Data obtained from GemScan1350. (© Copyright AcuTech Scientific, Inc. All rights reserved. Image reproduced with permission of AcuTech Scientific, Inc.)
AMERICAN LABORATORY 32 JUNE/JULY 2017
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