The lab may replace the loupe as treatments of diamonds and colored
AMERICAN GEMOLOGICAL LABORATORIES, DANIEL ARMSTRONG, THOMAS CHATHAM, SMITHSONIAN
gemstones increase.
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GEM istorically, handshakes and honor seal million-dollar gem deals around the world, but on the streets of Chanthaburi, Thailand, nothing is returnable, regardless of defect or misrepresentation. The best defense against fraud in this center of colored stone trade is paying with a postdated check. The average person cannot spot a fake, let alone a real stone that has been enhanced. Gemologists, appraisers, and jewelers routinely use simple tools—such as a jeweler’s loupe, microscope, or solutions of known refractive index—to reveal more about a stone. But as enhancements become more difficult to detect, even experts are increasingly dependent on analytical chemistry to defend the integrity of their trade against aggressive tampering.
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Rachel A. Petkewich F E B R U A R Y 1 , 2 0 0 3 / A N A LY T I C A L C H E M I S T R Y
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What is a gem?
AMERICAN GEMOLOGICAL LABORATORIES
Gems are cut and polished stones, such as diamonds, pearls, and colored stones, including sapphires, tourmalines, opals, and tanzanite. The term “fake” is reserved only for glass or plastic substitutes. Buyers may encounter synthetics, which are grown in a lab but have the exact same chemical formula, hardness, and specific gravity as the corresponding natural gems, or simulants, which look like their respective gems but do not have the same chemical properties. Cubic zirconia (zirconium silicate) and moissanite (silicon carbide) are examples of common diamond simulants. Even natural gems may not be pristine. Poor-quality materials may be treated to garner a higher price. Heating or irradiation may improve color and clarity by altering the internal chem-
Creating clarity. Natural cracks and fissures in emeralds are often filled with oils and resins to increase the value of a stone. Over time, the fillers can dissolve from the stone or, as shown here, become cloudy.
istry of a stone, but these treatments can make some stones more brittle or result in an undesirable color change. For example, many pearls and some colored stones were unknowingly altered when the U.S. Postal Service began irradiating mail to eliminate potential anthrax attacks. Adding oils, resins, or waxes to mask significant fissures and cracks will improve a stone’s clarity and increase its price tag. Nevertheless, some synthetics and treatments have become acceptable to consumers—provided they know what they are buying. Although professional gem organizations have established guidelines for stone certification, the nomenclature that describes certain newer treatments remains a hotly contested subject. Gemologists may use scientific analysis to determine chemical structure and composition, but the traditional methods to evaluate stones don’t use modern technology and are still subjective. For example, gemologists use small, natural im72 A
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perfections called inclusions and marketplace comparisons to grade and certify stones. “Gemology—in comparison to other industries—is still in the dark ages,” says Cap Beesley, president of American Gemological Laboratories in New York City. His lab developed a series of seminars that introduce more advanced instrument technologies to the gemological community, so that people start thinking beyond the traditional microscope, refractometer, and polariscope.
Analyze this Labs may be foreign territory for gemologists, but gemstones are not strangers to analytical chemists. For example, corundum— the basic mineral name of the Al2O3 structure for ruby and sapphire—is a key component of electronics and lasers used in military equipment and scientific research and one of the world’s 20 most studied materials. Scientists have always synthesized stones for lasers. As the techniques for creating these stones improved, and the defense research programs collapsed in places like the former Soviet Union, jewelry became a secondary market for synthetic gems, recalls Daniel Armstrong, a chemist at Iowa State University. Despite all the study, there is still a lot to learn about synthesizing stones. Growing gem crystals is still a bit of “black art”, because knowing the stoichiometry of one gem is not necessarily the recipe for creating another, according to Thomas Chatham, president of the prominent U.S. synthetic gemstone company Chatham Created Gems, Inc. Geologists and solid-state chemists can explain some mineral formation and colors with established theories, but they use analytical techniques to learn more. Transition-metal activity governs most mineral chemistry in accordance with intervalence charge transfers, element substitution, and crystal field theories. Therefore, temperature and pressure strongly influence color and clarity as a colored stone develops or is chemically altered. “In gemological methods, modern high-tech methods are certainly applicable and absolutely needed—IR, UV, luminescence, energy-dispersive X-ray fluorescence, radioactive counting technologies,” says George Rossman, a mineralogy professor at the California Institute of Technology. “In an academic environment, [we] are not approaching the everyday problems of the gem industry but more the fundamental basics of color in minerals from a [purely] scientific point of view, and we need even a higher level of sophistication.” Adapting nondestructive analytical techniques to less-than-ideal situations has been a challenge for all. “People who owned gemstones objected to grinding them to a powder or dissolving them in solution for a good analysis,” so UV–vis, IR, and Raman data collected with standard equipment is precluded, says Armstrong. Gem thickness, facets, and jewelry settings mean that light “doesn’t necessarily travel in a nice straight line from the light source to the detector, so you have to learn to live with these things and compensate for them,” he adds. He and Beesley collaborated on work with near-IR and a diffuse reflectance FTIR device that helped consistently detect polymers, oils, and waxes
used for filling imperfections in notoriously fragile beryl minerals, such as emeralds. Although methods such as elemental analysis, secondary ion MS (SIMS), laser ablation, and electron microprobes can be destructive, they are also used to help establish composition.
The “corundum conundrum”
THOMAS CHATHAM
Tied to a history of myths and legends, certain stones are believed to ward off evil spirits and temptation or bring good luck while others can end up worth more in memories than money. Sapphires are the latest example. Rubies and sapphires have been heated for centuries, but primitive furnaces did not produce enough heat to significantly change them. Furnaces today are much more sophisticated, says Richard Hughes, an author and gemologist for gem wholesaler Pala International in California. For example, in the mid-1960s, some gem treaters from Thailand discovered that previously worthless stones lining driveways in Sri Lanka could be heated to near melting and brought from “brand X to killer colors” with “zip to significant value,” adds Beesley. “It was an alchemist’s dream.” Treatments are becoming more complex because, as Chatham says, “In the last decade, [treaters] brought in the chemists.” Rubies are red—and rare—primarily due to a small amount of chromium in the crystal lattice. Iron and titanium content are responsible for the color of blue sapphires, and other trace combinations explain other, less-common colors of sapphires. Rossman
Diamonds in the rough. Synthetic stones, such as these rough white, pink, and yellow diamonds, have all of the chemical and physical properties of a natural stone, but were grown in a lab.
says heating intensifies the color of a blue sapphire by “tailoring the ratio of Fe(II) to Fe(III) to optimize the intervalence charge transfer.” TiO2, which occurs naturally in sapphires, can precipitate out to form myriads of microscopic crystals in the Al2O3 structure, making the gem appear cloudy, milky, or turbid.
However, he explains, if you heat the stone to 1600–1800 °C, the TiO2 will dissolve into the Al2O3 structure, and “it could be 100 (b) million years before [reprecipitation] is noticeable.” Although people still take their chances with primitive furnaces, stones are frequently heated in a sensor-controlled environment because color and clarity will (c) change with variations in oxygen partial pressure and the ratio of iron oxidation states. Diffusing any foreign material other than hydrogen into a stone during heating creates controversy in gemology. Hughes Inclusions in colored stones. dubbed research into Gemologists and appraisers look for the latest treatment natural imperfections called incluthe “corundum cosions to grade gemstones. (a) These needles are never seen in synthetic nundrum”. It started rubies; (b) heating would remove this with padparadscha, an “silk” from a natural sapphire, but extremely rare, pinkmight change (c) the solid, liquid, and ish-orange sapphire gas phases of this emerald. named for the exotic color of lotus blossoms, but has spread to other colors of sapphire. Vast quantities of orange stones started to appear in Thai and Japanese markets in 2001. Looking at a cross section, Ken Scarratt, director of the American Gem Trade Association (AGTA) Gemological Testing Center, noticed a yellow rim on a pink stone, which explains why it appeared to be orange. Madagascar pink sapphire retails for approximately $1000 a carat, whereas natural padparadscha can sell for up to $25,000. James Shigley, the director of research at the Gemological Institute of America (GIA) based in California, says the labs “have been trying to understand the treatment process in a way that there is some scientific integrity,” so they called on corundum expert John Emmett, who, with Troy Douthit, started a small company called Crystal Chemistry in Brush Prairie, Wash. after retiring as the associate director responsible for laser development at Lawrence Livermore National Laboratory. Emmett was confident that “the only way you can get those rims is by diffusion [of a foreign element like beryllium], and difF E B R U A R Y 1 , 2 0 0 3 / A N A LY T I C A L C H E M I S T R Y
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(A) THOMAS CHATHAM; (B) AND (C) AMERICAN GEMOLOGICAL LABORATORIES
(a)
RICHARD HUGHES/PALA INTERNATIONAL
Refractive index solutions reveal treated sapphires. Natural orange sapphires should “disappear” when immersed in diiodomethane because the naked eye cannot distinguish between two materials of the same refractive index. This cross section of a treated orange sapphire viewed in immersion is typical of those that have created the recent controversy. Here, the obvious orange rim exactly conforms to the surface contours of the gem surrounding a pink core, but the orange color may penetrate all the way through some stones.
Hughes warns that the “diamond industry will experience some serious challenges distinguishing treated from pristine stones over the next 5–10 years—including everything from the wedding ring to the dramatic stones in museums.” Jewelers can use thermal conductivity testers to distinguish real diamonds (tester shows a green light) from simulants (red light), but not from treated diamonds (green light). The U.S. Federal Trade Commission (FTC) requires that all synthetics and treatments be labeled by sellers and maintains guidelines for gems (www.ftc.gov/bcp/guides/jewelgd.htm). However, Robin Spector, a lawyer with the FTC, says they have received very few consumer complaints about misrepresentation in marketing and advertising that involve jewelry. “The gem business is not all about scandals—it is about bringing very rare and beautiful things to market and has a history going back thousands of years,” reminds Hughes. But even if relatively cheap analytical techniques were available to verify quality, the reputation of certain stones may not be saved. For example, so much blue topaz was treated in the past that the bottom fell out of the market. Now, the trade just assumes that all blue topaz is treated, so the stones are cheap. Gemologists say the Internet helped quickly alert the trade to the scientific findings for the orange stones, though it did not help the dealers who paid huge amounts of money for stones that no one will buy.
More treaters than testers fusion is a dirty word in the gem business.” He based his conclusion on previous experience using SIMS results to explain the natural occurrence of orange spots in blue sapphires from Montana; in that case, divalent impurities exceeded tetravalent impurities and essentially trapped holes that would absorb light differently than the rest of the stone. First, to prove beryllium could diffuse into corundum and create a trapped hole center that makes a pink stone look orange, he created yellow rims on pink sapphires. Second, SIMS tests of the rims on sapphires found in Thai and Japanese markets showed 10–30 ppm of beryllium. Emmett says beryllium could occur naturally in any sapphire, but even 1 ppm “is an extraordinary find,” so the results confirmed that “millimeter-type depths” of yellow on pink stones was an intentional treatment. After spectroscopy results concurred, gemologists were convinced about the treatment process and began to alert the trade. Thai traders who sold the stones initially denied intentional diffusion, but Hughes says “[the labs] have no question about it.” American and European markets were largely unaffected because buyers were alerted in time. However, orange stones are much more popular in Japanese markets. They were flooded with treated stones that had been mistakenly certified as natural. Hughes says people have incentive to create new treatments if the trade puts treated stones alongside high-grade, natural stones in the market. He is one of many gemologists who fear analytical chemistry may be the only way to examine diamonds and colored stones in the future. Even some large companies are trying to make the most of undesirable yellow and brown diamonds, which are colored because nitrogen gets trapped during formation. 74 A
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Shigley says requests for analytical confirmations from gemologists are increasing. “We find that there are more and more subtle differences to check for to identify synthetic and treated gems.” Although Emmett advised AGTA and GIA to do systematic SIMS and laser ablation studies to better understand the trace-element chemistry in the sapphires, he says, “There is no money to do gemological research like there is if you are working on laser materials.” Hughes says even raising $15,000 to test the orange stones was “very hard.” Emmett will likely pay for additional laser ablation and inductively coupled plasma MS work “just to satisfy [his] curiosity.” Armstrong says government funding is limited because “it is not considered ultrabasic science, or it doesn’t have a direct effect on health or energy or defense.” Beesley adds, “In gemology, there are very few individuals with deep pockets interested in funding even basic research to stimulate a deeper understanding of the gem sciences.” “Every jewelry store in the nation cannot have a quarter-milliondollar IR spectrometer with cryogenic accessories or scanning electron microscopes,” Rossman says, so jewelers need quick, convenient analysis using refractive index, density measurements, and conductivity. Beesley and Armstrong believe that the gem industry can probably afford miniaturized analytical instruments and plan to create specialized prototypes. Beesley says, “No matter what new treatments are developed, the answer is trapped in the material—it is simply choosing the technique and methodology to identify it.” Although the concept may ruin the mystique of gem trading, a field instrument should protect buyers better than postdated checks. Rachel Petkewich is an assistant editor of Analytical Chemistry.
