Raman spectroscopy, gems

Raman spectroscopy is evaluated as a non-destructive method for identifying gemstones from vibrational spectra governed by chemical constitution and crystal structure. The principles of optical absorption, Rayleigh scattering and Raman scattering are outlined, with molecular stretching and bending vibrations used to explain the relationship between infrared and Raman spectra. Laser excitation produces weak scattered light shifted in frequency from the incident beam; these shifts provide characteristic information about atomic bonding. A practical advantage is that only one accessible surface is required, allowing examination of mounted stones and, in suitable cases, identification of both components of a doublet through its table facet. Spectra were obtained at room temperature using an argon-ion laser operating at 514.5 nm and a backscattering arrangement. The samples included natural and synthetic diamonds, several diamond imitations, natural aquamarine, synthetic emeralds, topaz, yttralox and two doublets. Natural and synthetic diamonds gave indistinguishable spectra dominated by a sharp band at 1332 cm⁻¹, demonstrating identification of diamond without establishing its origin or colour variety. Cubic zirconia, yttrium aluminium garnet, gadolinium gallium garnet, strontium titanate, rutile, sapphire, spinel, quartz and silicon carbide showed distinctive spectra permitting material identification. Differences in relative band intensities among cubic zirconias were attributed to crystal orientation and polarisation rather than differences in stabiliser composition. Beryl, topaz and yttralox could also be distinguished, but the spectra of aquamarine and synthetic emeralds were qualitatively similar. Observed intensity differences among the beryls were insufficiently established to support reliable separation and might likewise reflect orientation effects. A fabricated natural diamond–cubic zirconia doublet yielded a superposition of the spectra of both components despite illumination and observation through the diamond surface. In a sapphire–strontium titanate doublet, strong fluorescence from the adhesive masked the component spectra, illustrating an important limitation. Glass and plastics could also produce interfering fluorescence. The method therefore offers convincing identification of many gem materials and useful detection of composite structures, but generally does not establish natural versus synthetic origin. The author considers the contemporary equipment cost, approximately US$20,000 or more, too high for routine gem testing, while recognising its potential for resolving particularly difficult identification problems.