gemstones

Contents


  Raman Spectroscopy and the Analysis of Gemstones
gemstones
Raman Effect/Raman Scattering       C.V. Raman       Raman Spectroscopy       Analysis of Gemstones       Classroom Assignment       Works Cited

~ 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
emeraldtanzanite     garnet    
Figure 1                                                                                             Figure 2
                                        Emerald - Solid green spectrum                                    Square cut - top
                                        Tanzanite - Dashed spectra                                          Oval - middle
                                                                                                                                     Round - bottom


Ruby

ruby
Figure 3

Raman Identification of Genuine and Fake Gemstones

Figure 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)
diamondimposters          citrinetopaz
                                                                          Figure 4                                                                     Figure 5
                                                
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)
quartzite         emerald resin
                                                                     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)

monazite
Figure 8
Bottom - Untreated
Top - Heat treated









Contents

Raman Effect/Raman Scattering       C.V. Raman       Raman Spectroscopy       Analysis of Gemstones       Classroom Assignment       Works Cited