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March 2001
Vol. 10, No. 03, pp 38–43.
 
 
 
Today's Chemist at Work
Focus: Trace Metal Spectroscopy

FEATURE

Precious Provenance

Spectroscopy is key to tracing metals and mines in the judging of gemstones.

opening artWhat is value? Medieval alchemists once dreamed of finding a process to turn lead into gold. Had they been successful, we would not now be looking to Fort Knox for the yellow metal, but rather to the local hardware store. Value lies in either utility or scarcity. What makes a gemstone valuable? Other than diamond dust for grinding purposes and rubies for lasers, it is difficult to find much utility in pretty rocks. Scarcity is the key.

And in a world in which modern synthetic gemstones become ever more sophisticated, where “enhancements” of colored gems are becoming ubiquitous and accepted, provenance becomes all. Did your luxury purchase originate in a mine deep in the heart of Central America, or the bottom of a silty river tributary in Africa, or perhaps even a flask in a laboratory in Chicago or Minsk? Was it a low-quality product enhanced by heating or irradiation to bring out luster and colors not previously there? Is it one gemstone masquerading (with a little help from makeup) as a jewel costing 10 times its actual worth? Or was it simply cut and polished, a pristine product of Mother Nature’s beneficence?

Sophisticated analytical instrumentation is often all that stands between the public and the chemist’s new artistry of gemstone creation and enhancement. Spectroscopy often provides the only means for separating the fake from the fabulous. And as gemstone technology becomes increasingly advanced, the tools for detecting its art must also become increasingly sophisticated. Today we are no longer in the era of handheld refractometers, UV lights, and color spectrometers. Raman spectroscopy, ICP-MS, and especially energy-dispersive X-ray fluorescence (EDXRF) have entered the fray as necessary tools to identify provenance through sophisticated trace-metal and/or isotope analysis.

Gemstone Qualities
Gemstones are mineral crystals with properties derived from the lattice matrix of the main element(s) involved (e.g., diamonds are mostly carbon), any trace minerals present, and the mechanics of their formation—usually condensation, heat, and pressure. After the primary matrix and contaminants from a gemstone’s surrounding mineral environment condense into a localized accretion, high temperatures and pressures combine to transform the materials. Superman crushing a lump of coal into a diamond in his hand may spring to mind. Although each matrix tends to have a basal color, variations in color are caused by the presence of impurities, general trace metals that were caught up in the formation process. Metal ions such as V3+, Cr3+, Mn2+, Mn3+, Fe2+, Fe3+, Ni2+, Cu2+, and UO22+ are responsible for the colors of most common gemstones and minerals.

Various gemstones also have mineral or metallic inclusions that affect their appearance dramatically and are often used for determining identity or provenance.

Enhancements?
As detailed by the U.S. Federal Trade Commission, a wide variety of techniques are used to improve (and sometimes counterfeit) the appearance of natural gemstones. These include heating to lighten, darken, or change the color of some gems; irradiation—often gamma or electron irradiation—to add more color to colored diamonds, some gemstones, and pearls; impregnating gems with colorless oils, wax, or resins to improve the clarity and appearance (an almost ubiquitous practice with emeralds); fracture filling by injecting colorless plastics or glass; dyeing, which can enhance or provide uniformity to the color of certain gems and pearls; and bleaching, which lightens and whitens some gems, including jade and pearls. Consumers must be informed of such alterations if the treatment is not permanent or if treated materials require special care. If such practices are not reported to consumers when they should be, or if new owners want to know to what extent their stone differs from its “natural” state, sophisticated instrumentation must be put to work.

Recently, a high-pressure, high-temperature technique developed by General Electric has shaken up the diamond world by proving able to transform diamonds of low color quality—even brownish stones—into gems with brilliant clarity and enhanced color, including the very valuable blues, pinks, yellows, and yellow-greens.

By Any Other Name Would Smell. . .
One peril of the gem market is the ignorance and avarice of the modern world traveler who, tempted to get a tremendous “bargain” or to buy a pricey investment-grade souvenir, falls for a common scam and buys lesser-quality or phony products, thinking they are real.

According to Kevin Coffey at Corporate Travel Safety (www.corporatetravelsafety.com/gemstones.html), purchasing gems overseas is a dangerous enterprise, in part because of labeling “name games” in which misleading stone names are used to dupe consumers into thinking a synthetic stone is real, or one type of gemstone masquerades as another. Coffey lists 32 of these common “misnomers”—although, in truth, many of these can be considered legitimate names and may not be considered false advertising in countries without a strong watchdog like our own Federal Trade Commission.

Among the problem gems encountered by the uninitiated are Japanese Amethyst (synthetic), Herkimer Diamond (colorless quartz), Chatham Emerald (synthetic), Cape Ruby (garnet), Red Sea Pearl (coral), and Madeira Topaz (quartz). As for “authentic” fakes, in the modern era, as this article shows, without prior analytical analysis and a trustworthy certification to that effect, it is impossible for the average consumer, and often even the unaided expert, to be sure of getting the real McCoy—even if they think it’s called Mack Coy.

New Facets
Even more problematic to consumers and the corner jewelry store gemologist is identifying the new synthetics. Many of the most popular gemstones have synthetic versions available that can often be used to fool the unwary consumer (see box at right, “By Any Other Name Would Smell...”). Cubic zirconium, a synthetic diamond, is easily distinguishable from the real thing by the thermal testers used routinely in a jeweler’s practice. But the latest “false” diamond, moissanite, requires more sophisticated methods, including spectroscopic analysis. Trace-metal analysis in particular has proved invaluable in identifying some synthetics.

There is, of course, a flourishing and legal trade in synthetics. The Japanese company Kyocera Corp., for example, markets 12 varieties of created gem material, including synthetic emeralds, several colored sapphires, and white and black opals. Some producers of synthetics and enhanced gems have gone so far as to develop reliable ways of discerning their products from genuine gems and have even incorporated chemical identifiers to make it less likely for unscrupulous individuals to pass them off as “real”.

Tracing Metals
For the most part, nondestructive methods are preferred for gemstone trace-metal analysis. A variety of such techniques are available, including Raman and FTIR spectroscopy. Both of these methods are useful for distinguishing natural versus synthetic gemstones by identifying the composition of inclusions (nonmatrix material depositions common in many gems). According to Gary Roskin, in the October 1998 issue of JCK Magazine, “Until a decade ago, it was difficult to identify certain synthetics because the visible light spectrum of a synthetic gem matches that of a natural stone. Now that there’s a way of detecting infrared spectra . . . the synthetic gem world has become an open book.”

Among the most sensitive and popular of the nondestructive spectroscopic techniques used for trace-metal determination is EDXRF. In this technique, X-rays excite the gemstone to fluoresce and the fluorescent line spectrum indicates which chemical elements are present.

According to Muhlmeister and colleagues in a 1998 article in Gems & Gemology, synthetic rubies can only be distinguished from their natural counterparts by analysis of trace metal chemistry by EDXRF. (Laboratory-grown crystals differ markedly from the variations seen in the “wild”-grown gems.)

EDXRF can also be used to differentiate freshwater from saltwater pearls on the basis of the greater concentration of magnesium present in the former. It can detect a variety of the gem enhancements described above because many of these alter the chemical elements present. Differences from “natural” gems can be detected quite easily to reveal whether chemical impregnation or staining has occurred. A variety of vendors provide the appropriate instrumentation for all such spectroscopic analysis. Researchers at the Swiss Foundation for the Research of Gemstones (SSEF), for example, use a Tracor X-ray Spectrace 5000 EDXRF spectrometer with special software to detect fissure fillings in diamonds; identify pearls as to salt or fresh; and determine the probable origins of rubies, sapphires, alexandrites, and emeralds.

Tracing Mines
Identifying the exact origin—the mine or at least the country—of a particular gemstone helps to determine its worth, as some mines or producer countries are associated with increased financial value. Consumers or gem dealers may also want to know a gems origin because of concerns about smuggling.

Raman spectroscopy is especially important for fingerprinting inclusions. Besides being useful for separating naturals from synthetics, particular inclusions are common to geographic regions and so they can be used to identify the country of origin, or in some cases, the actual mine from which the gem was obtained.

For gems that do not commonly contain significant inclusions, such as diamonds, other methods must often be used. In some cases, as for rubies, EDXRF has helped identify the geological environment in which the stones were formed, to aid in pinpointing their origin.

Laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS) has also proved useful in the area of identifying gemstones, despite the fact that it is a “destructive” technique. A laser beam is used to vaporize an extremely small sample of the material (in this case, a gemstone). The ablated sample is carried by a stream of inert gas, usually argon, into a high-temperature field, causing dissociation of molecules and ionization of atoms. The MS identifies and quantifies elements in terms of mass and charge. Some 65 elements and their relative amounts can be detected even when present in only a few parts per billion.

Determining the country of origin can have a serious political purpose when it is used to prevent the unregulated sale (essentially smuggling or arms-dealer “laundering”) of what are generally known as “conflict” stones, most often diamonds.

According to a UN-financed report by Global Witness, DeBeers has defined conflict diamonds as “diamonds which originate from areas in Africa controlled by forces fighting the legitimate and internationally recognized government of the relevant country.” No longer simply buying up diamonds from conflict zones to prop up the world market, the diamond cartel has moved to cooperate with efforts being made worldwide to prevent such trade.

One example of governmental efforts to regulate the trade is the work of The Royal Canadian Mounted Police. This group has researched the use of LA ICP-MS technology as an identification tool and is trying to obtain cooperation worldwide from the diamond industry in order to collect data that will establish unique identifying characteristics of diamonds from various mines.

Until a representative database is established, there is no certainty that such identification can be routinely done. Diamonds are notorious for deriving their characteristics from the deeper mantle in which they were formed, and often are little affected by the location in which they ultimately are found. In the case of conflict stones, conclusive proof of origin may not be necessary as long as it is possible to determine that a sample did not come from a particular location.

Determining the origin of other gems, such as emeralds—that are formed comparatively “locally” in geological strata—is somewhat easier, and this is another area in which ICP-MS has proved useful. For example, G. M. Pulz and colleagues used ICP-MS in combination with FTIR and Mössbauer spectroscopy to identify unique chemical signatures for emeralds derived from the Campos Verdes–Santa Terezinha Mining District in Goias, Brazil. The gems contained measurable amounts of Fe2+, Fe3+, Cr, V, Sc, Ni, and Zn as well as Ba, Sr, Rb, and Pb.

The Lure of Luxury
As long as gems remain signs of wealth, objects of investment, tokens of beauty and affection, and historical artifacts, there will be an increasing demand for them, and an increasing temptation to synthesize, enhance, and even forge jewels of quality and character. It is an arms race between two branches of science, sometimes cooperative, sometimes at odds. The debars monopoly on diamonds helps prop up the market and keep diamonds “valuable”, although this is a far more difficult task today than it has been in the past, because global conflicts and economic chaos in major diamond-producing regions threaten the group’s control. A diamond glut is a constant risk. And in the laboratory, new synthetics and enhancements and the creation of “natural” gemstones may well have the potential to one day make certain gems as common as cut glass. Will their beauty alone and their traditional associations with love or luck or health be enough to provide them with value? Or is beauty not in the eye of the beholder, but in the calculation of the investor? In the Middle Ages, in Europe, sugar was once the sole province of kings. And pepper, paprika, and cinnamon were once worth more than gems and gold. Familiarity—and availability—breed contempt.

Further Reading

  1. Gemological Institute of America home page: www.gia.edu (includes access to online materials from the quarterly journal Gems & Gemology).
  2. On instrumentation for gemology: The Swiss Gemological Institute, the Swiss Foundation for the Research of Gemstones (SSEF) home page: www.ssef.ch.
  3. On conflict, diamonds, and ICP-MS: www.oneworld.org/globalwitness/reports/conflict/conflict.html.
  4. Anderson, B. W., Payne, J., Mitchell, R. K., Eds.The Spectroscope and Gemology, GemStone Press: Woodstock, VT, 1998.

(All Web sites accessed March 2001.)


Mark S. Lesney is a senior editor of Today’s Chemist at Work. Send your comments or questions regarding this article to tcaw@acs.org or the Editorial Office 1155 16th st N.W., Washington, DC 20036.

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