Contents Raman Spectroscopy and the Analysis of Gemstones |
~ Analysis of Gemstone Using Raman
Spectroscopy ~
In
the recent past, the fate of the international gemstone trade has been
jeopardized, mainly due to human modifications and synthesis of
gemstones to be sold as impersonators of precious or semiprecious and
valuable stones. Even analysis by qualified jewelers is sometimes
not adequate enough to differentiate between genuine and fake
gemstones. Some of the most sophisticated techniques fail to
recognize diamonds that have been modified. As such, it is
imperative to the gemstone market that a reliable method of tracking
genuine and fake stones be utilized. Raman spectroscopy is one of
the most ideal techniques available for this particular type of
analysis because it is non-destructive and it does not require
additional sample preparation. As an alternative to other
analytical methods for gemstones, such as refractive index, specific
gravity, and absorption spectra, Raman spectroscopy is desirable
because it is less time consuming, it is a more refined analysis for
gemstones of similar properties, it is effective even when the surface
of the stone is rough, there is less restriction on the size of the
stone, and the stone can be analyzed even when set in jewelry.
An important and popular field in gemology is the treatment of gemstones with the purpose of improving their appearance. Some of the most common treatments include (19): -Heat Treating – enhances clarity or creates color in a stone Examples: Citrine is almost always heat-treated amethyst. Zircon achieves its clear white appearance through artificial heating. Sapphires are heated to characteristic pinks and blues. Upon heating, rubies lose their tint of purple. -Dying – achieves color change Examples: Pinks, purples, organges, and blues of agate are obtained through dying. -Irradiation –color treatment Examples: Topaz can come in shades of blue through irradiation. Diamonds were the first gemstone to be color treated by radiation. -Stabilization – traditionally this meant filling the stone with natural oils, however, modern synthetic resins (such as Opticon) that are more permanent are used now Examples: Opals and emeralds are often stablilized due to frequent fractures. -“Creation” – synthetically prepared stones Examples: Cultured pearls (which are in fact genuine) have a plastic center instead of sand. Real ruby, sapphire, diamond, emerald and sapphire crystals are synthesized in labs. As with other forms of spectroscopy, Raman spectroscopy is capable of producing a “fingerprint” for a particular molecule, or for a particular gemstone. Raman spectra result from light-induced changes in the polarizability of a molecular bond during vibrational motions (article). These spectra are capable of distinguishing between genuine gemstones and their imposters, and they are capable of recognizing stones that have been altered (ex. stabilization, irradiation). Below are spectra comparing various genuine and fake stones, in addition to altered stones. This comparison allows for us to distinguish very easily between gemstones. Raman Spectra as Gemstone Fingerprints (20) Figure 1 shows the distinction between the spectrum of an emerald and that of tanzanite. Figure 2 shows that even with different cuts of a gemstone (in this case, a garnet), the Raman spectra will be identical. Figure 3 shows the Raman spectra of 3 different ruby gemstones, all of which show the same characteristic peaks. Emerald & Tanzanite Cuts of Garnet Figure
1
Figure 2
Emerald - Solid
green spectrum
Square cut - topTanzanite - Dashed spectra Oval - middle
Round - bottom
Ruby
Figure 3
Raman Identification of
Genuine and Fake GemstonesFigure 4 shows the differences in Raman spectra for a diamond and its most common impressionists. Figure 5 shows the difference between the spectra of a cheap stone of citrine which is oftentimes passed off as a more valuable topaz gem. Diamond Forgories (21) Topaz Forgories (20) Red - poly methyl methacrylate (PMMA) Citrine - bottom Blue - real diamond Topaz - top Purple - crystal glass Raman Identification of Treatments Figure 6 shows the difference in spectra between pure white quartzite and artificially colored quartzite. Figure 7 demonstrates the difference in fingerprints due to the injection of an emerald with Opticon epoxy resin, a common articifial resin to enhance stabilization of a gemstone. Figure 8 shows the effect of heat treatment on the Raman spectrum of monazite. Artificial Coloring of Quartzite (22) Stabilization of emerald with Opticon epoxy resin (22)
Figure 6
Figure 7
Bottom - pure white quartzite Bottom - unaltered emerald Top - artificial coloring Middle - Emeral altered with Opticon epoxy resin Top - Opticon epoxy resin Heat
Treatment on Monazite (22)
Figure 8 Bottom - Untreated Top - Heat treated |