Published: 19 September 2017
In recent years, the gemstone market has been flooded with stones of questionable origin. Frequently, even thorough analysis by a qualified jeweler cannot unequivocally reveal whether a gemstone is genuine or fake. In the worst case, even sophisticated analytical methods struggle to differentiate modified diamonds, causing considerable concern to the international gemstone trade. Raman micro-spectroscopy is an ideal method for the examination of marketable gemstones. The lack of sample preparation and the non-destructive nature of Raman analysis make it ideal for the analysis of even high-value gems. Plus, the micro-Raman study of a stone provides a unique record for identification purposes. We will discuss the variety of Raman spectra that can be obtained from different families of gemstones, comparing and contrasting spectra from genuine and artificial materials.
Gemstones and semi-precious stones have been modified for centuries to make them more colorful, more eye-catching, and easier to work with. One such technique is heat treating (the most common) to enhance, clarify or create color in a stone. Amethyst is heated for citrine. Zircon is heated to clarify the stone to clear white. Sapphires are heated to get amazing pinks and blues. Rubies lose a purplish tint while iolite may be turned a deep blue. Heat treating can also be used to enhance the ‘color change’ of gems such as tanzanite. Dying of stones is also a very common practice. Agate is dyed to get pinks, purples, orange and blues. Chalcedony is also dyed; black chalcedony sold as onyx. Irradiation is another common treatment. Topaz is currently the most commonly irradiated gemstone (to get various shades and tones of blue). This is also how one obtains fabulously colored diamonds. In fact, diamond was the first gemstone to be color treated by radiation. (Figure 1) Another gemstone modification is stabilization. Stabilization was traditionally accomplished by filling the stone with natural oils; however, modern synthetic resins such as Opticon are now being used. Resin filling is often more permanent than the use of natural oils. Opals are often stabilized and emeralds have a long history of fracture filling due to their popularity and a tendency to be highly fractured or contain multiple inclusions.
The most controversial of all the modification techniques is the ‘creation’ of gemstones. Cultured pearls are genuine but are created by using a center of plastic or mother of pearl, rather than sand. Still a pearl, just helped to grow by human intervention.
Laboratory grown crystals of ruby, sapphire, diamond, emerald, and star sapphire are real semiprecious stones. They just weren’t grown in the earth. So what is the answer: Real or Fake? This argument can be discussed with all sides being technically correct, but it is not the most important information. From a lapidary or jeweler’s point of view, the most important topic is proper disclosure. Does the buyer know up front that the stone he is purchasing has been ‘helped along’ by the human touch?
Gems are often examined by trained personnel using optical microscopy and other methods. In some well-studied cases like diamond, these techniques will usually suffice. However, imperfections can be readily filled with synthetic materials or the stone can be processed to alter the color and increase market value, with the unsuspecting consumer convinced that he has purchased a stone of greater value. With lesser gemstones the analytical techniques are much less established and more reliant on long experience with mineralogical identification methods. Raman spectroscopy however provides an ideal method for the examination of gemstones and semi-precious stones. With the ability to microscopically examine both loose and mounted stones, Raman can distinguish not only real versus artificial gemstones, but can also discriminate those that have been adulterated in addition to providing details of the alteration.