An extensive variety of gem materials are available in the jewelry marketplace at present. However, in some cases such materials are sold with inaccurate information on their identity.
The examination of gemstones requires an effective, easy-to-use analytical method. Raman microscopic technique offers unique possibilities for non-destructive and non-contact gemstone analysis at submicron level. It is particularly topical in case of very small gems which are mounted in pieces of jewelry.
This application note by SOL instruments experts describes the use of confocal Raman spectroscopy for the identification of authentic materials on the gem market.
Precious stones (diamond, emerald, ruby, sapphire, pearl, etc.) as well as semiprecious stones (alexandrite, chrysolite, turquoise, garnet, topaz, amber, coral, etc.) are very popular and extensively used in the jewelry industry.
The Raman spectra of individual gem materials are shown in Fig.1-9. Also, the Raman spectra of gemstones from the RRUFF spectral database are given in Fig.1-9 additionally. Each of the measured materials has the particular and clearly identifiable Raman spectrum.
Diamond is the most valuable gem. Zircon and cubic zirconia, the high-usage imitations of colorless diamonds, can be passed off as the more expensive diamonds. Such imitations are very much alike, but their Raman spectra are completely different (spectra are displayed in Figures 1-3).
Diamond (Fig.1) shows a strong Raman band at around 1332 cm-1 (C-C stretching mode), which is absent in the cubic zirconia (Fig.2) and zircon (Fig.3). The spectrum of zircon shows multiple Raman peaks at around 350, 432 (Si-O bending modes), 969 and 1003 cm-1 (Si-O stretching modes).
The next important precious stone is ruby, a variety of the mineral corundum (Fig.4).
Prices of rubies are primarily determined by color. The most valuable stones are red. Pink, orange, and purple are the normal secondary hues. Imitation rubies are also marketed. Red spinels and colored glass have been falsely claimed to be rubies by unscrupulous gem dealers. Raman spectroscopy allows to distinguish genuine rubies and their imitations.
The Raman spectrum analysis can also help to easily identify tourmaline (Fig.5), nephrite (Fig.6), benitoite (Fig.7) and amber (Fig.8) from their substitutes. Additionally, the Raman spectra can separate real from faux pearls. The real pearls have very characteristic Raman spectra (Fig.9).
Conclusion
Confocal Raman spectroscopy is an ideal method for the analysis of gemstones. Due to its non-destructive technique Raman spectroscopy is very useful for gem merchants, gemologists, gem collectors and others.
Fig.1. Raman spectra (λех = 532 nm) of diamond.
Fig.2. Raman spectra (λех = 532 nm) of cubic zirconia.
Fig.3. Raman spectra (λех = 532 nm) of zircon.
Fig.4. Raman spectra of ruby.
Fig.5. Raman spectra of tourmaline.
Fig.6. Raman spectra of nephrite.
Fig.7. Raman spectra of benitoite.
Fig.8. Raman spectra of amber. A Reference Raman spectrum of amber (from Howell G. M. Edwards, Analyst, 2004, 129, pp. 870-879) is shown on the right.
