New Gem Identification Techniques
This is an exciting era for gemologists. High-tech analytical techniques now make it possible to identify trace elements, treatments, and origins of colored stones that could only be guessed at decades ago.
For example, when the gem world was rocked last fall by mysterious, processed orange sapphires coming out of Chanthaburi, the Gemological Institute of America (GIA) relied on two of these high-tech procedures – Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Secondary Ion Mass Spectrometry (SIMS) – to discover what elements could be causing the surface color.
Paulo Vasconcelos, a professor at the University of Queensland in Australia, has used high-tech techniques for a decade to study color in quartz and chrysoprase. Employing a technique called Electron Probe Microanalysis (EPMA), be studied the iron content in ametrine and determined that, contrary to previous theories, the citrine zones actually contained more iron than the amethyst zones.
French researchers at the Center for Petrographical and Geochemical Research in Nancy, France, successfully applied SIMS technology in 2000 to determine the origin of nine emeralds from the Gallo-Roman era to the 18th century. In one surprising conclusion, several old-mine emeralds from the treasury of Nizarn of Hyderabad in India that were believed to come from Southeast Asia were actually traced to Colombia.
How do these techniques work?
LA-ICP-MS identifies elements based on their atomic mass. A small sample of the material is vaporized by a laser. The remains are ionized and sent into a mass spectrometer to identify each element. The results are output as a graph, or “spectrum,” with peaks indicating high concentrations of a particular element.
The LA-ICP-MS is useful for trace metal analysis, especially at very low levels of concentration. It might be used in gemology to determine the origin of a gemstone based on unusual elements or the pattern of elements in its chemical makeup.
Widely used in geological research, SIMS is similar to the LA-ICP-MS, but uses an ion beam instead of a laser. The surface of the sample is bombarded with a beam of energetic primary ions. Some of the atoms on the surface are knocked loose as secondary charged ions. These ions are accelerated and measured by a mass spectrometer, which produces a similar output to the LA-ICP-MS.
SIMS is the most sensitive of all the commonly employed surface analytical techniques. It has a more powerful mass spectrometer and better resolution than the LA-ICP-MS.
EPMA is used to identify major and trace elements in solid materials. The specimen is bombarded with a focused electron beam to cause emission of characteristic X-rays. The X-rays are measured by energy and wavelength dispersive spectrometers, which produce a graph or “spectrum” containing peaks unique to that material,
Different techniques work for different applications. SIMS and LA-ICP-MS are more sensitive than EPMA for detecting trace elements, but the EPMA is more accurate at identifying major element concentrations, says John Donovan, lab manager at the University of Oregon.
Which instrument researchers use depends on their needs. For example, when gemologist and geologist Mary Garland decided to study the origin of western Montana alluvial sapphire for her doctoral degree, “I needed a very sensitive instrument to detect trace elements,” she says.
She first used the LA-ICP-MS, but found several problems: The laser made visible holes in the samples, the results were hard to standardize, and the system was so sensitive that the error rate tended to be high.
Her tool of choice ended up being the proton microprobe, using a technique called Proton Induced X-Ray Emission (PIXIE). Similar to the electron microprobe technique, protons are fired at the stone and release a spectrum of X-ray energy. Compared to electrons, the protons are bigger, heavier, and the results are more easily defined, says Garland.
Similarly, when the GIA was attempting to identify the treatment done to the orange-pink sapphires coming out of Chanthaburi, multiple techniques were needed to narrow down the answer.
GIA first examined samples with standard techniques such as X-ray fluorescence, with limited answers. “We could see visible color concentration on the outside, but they were not uniformly colored,” says James Shigley, director of research at the GIA. That suggested that some coloring agent was used, but the big question was which agent.
The GIA next turned to EPMA, which did not show any systematic chemical variations related to color zoning. But EPMA is limited because it cannot easily detect low concentrations of elements, so the GIA tried the more sensitive LA-ICP-MS at Boston University.
“We cut orange sapphire in half and made a flat slice, and analyzed various spot locators from the outside edge to the center,” explains Shigley. “We were looking for trends in any of the light elements; we didn’t know what [element] we were looking for.”
The LA-ICP-MS detected variations in trace element concentrations of a number of elements from the surface of the stones to the center. To confirm these findings, the GIA then ran the same samples through a SIMS instrument. SIMS was able to detect and measure lighter elements not picked up by the LA-ICP-MS, particularly beryllium. In fact, beryllium showed the most pronounced variation from the rim to the center of the stones.
“Beryllium is not thought of as a coloring agent in sapphire. We’re not certain yet that beryllium is causing it, but it is contributing to it,” says Shigley.
Worth the Investment?
Gemological labs are still a long way from buying this high-tech machinery for routine use, instead renting use of the equipment at universities and private companies for specialized applications.
One of the biggest issues is cost. An LA-ICP-MS or SIMS unit can run from $150,000 to $300,000, and routine maintenance like replacing a laser can be another $25,000 to $30,000.
“The problem is that the LA-ICP-MS is very costly and operators have to be so specialized,” says Christopher Smith, director of the Gubelin Gem Lab in Switzerland. “You have to earn a lot of money to have the unit. It can provide additional valuable information to routine testing available to labs today, but the cost, operation, and size are prohibitive for most commercial labs.”
It’s difficult to justify the cost of the more advanced instruments, agrees C.R. “Cap” Beesley of American Gemological Laboratories in New York. Besides the cost of the machine, there is the salary of a skilled technician and the cost of operation.
“It’s impractical,” he says. “Super sophisticated [instruments] require a higher level of skill. That’s the constraining factor. You have $150,000 for the [equipment], $50,000 for a technician. It takes two hours to do one stone, in 10 hours, you do five stones a day, or 25 a week.”
With LA-ICP-MS, for example, the operator must be extremely skilled to make sure the stone is not destroyed. “You have to have a specialized background to run it because it’s so sensitive and actually vaporizes part of the stone,” says Smith. What’s more, gemologists are dependent on the technician’s expert interpretation of the data. “It’s like taking a Ferrari to a garage,” says Garland. “In so many instances, the people getting the data don’t have the tools to say, ‘So, does the data mean what I think?’ “
These instruments require not only an investment in money, but in time as well. Any study of the origin of stones requires an extensive database of gemstone characteristics to match the readings against – and for now, no centralized database exists,
“You could fingerprint a lot of gemstones with this [SIMS] technique, but it requires a lot of work to build up a database,” says Steve Novak, manager of the SIMS Group at Evans East Analytical Laboratory, where the GIA had its analysis done. “If someone just brings me a sapphire and says, ‘Tell me where it comes from,’ I haven’t built the database,” so the reading by itself would say very little.
But gemologists agree that the number-one problem with these techniques is their destructive nature – especially LA-ICP-MS, which leaves a hole in the gem that in some cases is visible with a loupe.
Garland balks at using LA-ICP-MS for just that reason. “With the ICP-MS, you have a visible hole, 20 microns deep in some cases…. If you’re working with gemstones, that’s a lot of weight. To me, it was, ‘Oh, my God.’ “
Times change, however. As technology improves, the cost of this equipment may come down, making it more affordable for routine use. And as techniques for gem treatment and growing synthetics become more advanced, gemologists will need better equipment to detect them.
“When I started 20 years ago, [techniques like infrared and Raman spectrometry] weren’t routine,” says Shigley. However, as treatment processes grow more sophisticated, more advanced techniques are becoming useful on a routine basis. “Some of the treatment processes or crystal growth are similar to processes in nature; you’re looking for subtle chemical or spectral features to identify, and you need analytically more sophisticated instruments.
And with experience, adds University of Queensland’s Vasconcelos, use of such equipment can become second nature. “They’re not as difficult to apply as people think,” he points out. “I don’t think that a jewelry store will be using them, but labs process lots of stones.”