You can almost imagine the day it happened. A foreign gem buyer sat down across the table from his source and said, “What have you got?”
“Here’s a beautiful Thai sapphire,” the supplier said, holding out a vibrant blue gemstone. “Flawless. Look at the color. You never see color like this.”
“No thanks,” the buyer said, not taking the gem. “I’m looking for something from Burma this trip.” The supplier thought fast. “Okay, sure,” he said, fishing out a sapphire from Sri Lanka. “I’ve got a Burmese stone right here. Just look at this color. . .”
And thus was born the demand for laboratory-issued gemstone origin reports.
But how does a laboratory determine where a sapphire is from? And why will two labs sometimes disagree on the origin of the same stone?
Determining the Origin of Sapphires
At its most basic, sapphire is just aluminum oxide, Al2O3 52.9 percent aluminum and 47.1 percent oxygen. But Mother Nature almost never sticks to the basics, and sapphire-bearing rock always contains other things, like iron, titanium, or various minerals. The precise mix of ingredients is unique to each deposit, and stamps the gems produced there with an unmistakable fingerprint.
At least, that’s the theory. In reality, the differences between “recipes” can be very subtle, and overlaps can occur from deposit to deposit. While some gems wear their heritage openly, others require careful examination – and in some cases, even that will not rule out all possibilities.
“In every geo-chemical environment there are tell-tale signs trapped during the growth process,” says Casper “Cap” Beesley of American Gem Laboratories (AGE) in New York. “Sometimes [identifying those signs is] like looking at your brother, and sometimes it’s like going to your 25th class reunion, and if weren’t those little pictures on the name tag, you wouldn’t recognize [the person].”
In order to render an opinion on the probable origin of a sapphire, gemologists look at a number of characteristics, like inclusions in the stone, growth structures, and chemical composition.
Conclusions will be based on all these factors, not on any single one.
“The greater the number of individual and characteristic properties found in a certain stone, the more reliable is a determination of origin,” writes Henry A. Hanni of the Swiss Gemmological Institute (SSEF) in Zurich, Switzerland, in a 1994 article in the Journal of Gemmology. “Valuable characteristics should not only positively testify a certain origin but at the same time exclude other possibilities.”
Inclusions are one of the first things gemologists look at when trying to determine a country of origin. “Many times there are certain unique types of inclusions that are only found or at least, have only been found to date from certain localities,” says Tom Tashey, president and CEO of Professional Gem Sciences in Los Angeles.
For example, a stone with included pargasite or tourmaline crystals is likely to be from Kashmir, since those crystals have only been identified in Kashmir sapphires. Uranpyrochlore inclusions are regularly seen in stones from Cambodia or southern Vietnam, but do not appear to occur in sapphires from Myanmar (Burma) or Kashmir.
Gemstones from certain locations often command premiums because the deposit is associated with stones of a particularly beautiful and unique appearance. That appearance can also be a clue to the stone’s origin. Kashmir sapphires, for example, are renowned for their unique “silk” – particles suspended in the crystal that scatter the light – which gives the gems a soft, velvety appearance. The presence of that unique silk can steer the gemologist to look for other evidence of Kashmiri origin.
Fluorescence can also provide a clue to the gem’s origin. Although sapphires from many of the world’s deposits are largely inert to long- and short-wave ultraviolet light, sapphires from Sri Lanka exhibit such fluorescence with greater regularity than other sapphires, says Beesley. Although not all Sri Lankan stones fluoresce, the presence of such fluorescence is one indication of the gem’s origin.
The way crystals grow in a deposit can also provide clues to a sapphire’s origin. “Sometimes the form of the crystals, whether they are rounded or sharp, is more typical of one locality or another,” says Tashey. The growth patterns seen under a microscope can also be diagnostic. For example, tight optical growth patterns are found regularly in Madagascan sapphires, less regularly in Sri Lankan and Myanmar stones, and not at all in Kashmir sapphire, says Beesley.
Color zoning can also indicate the sapphire’s origin. Sapphire crystals from Antsiranana Province in Madagascar, for example, often display blueviolet, greenish blue, and greenish yellow zones within the same crystal. Kashmir sapphires exhibit sharp-bordered zones of blue and milky white.
Technology has given labs a new tool to determine country of origin. Spectrophotometry and energy-dispersive X-ray fluorescence (see sidebar “Say What?”) can detect even trace elements in a sapphire’s surface, giving gemologists a window into the unique chemical mix a sapphire grew in. Although most sapphires will show the same basic composition, the strength of the concentrations of trace elements can indicate which deposit the sapphire came from.
For example, Cambodian sapphires, many of which are often touted as Kashmir “lookalikes,” often show very high concentrations of iron, while Kashmir sapphires typically have very low iron levels, says Beesley. Absorption spectra produced by these technologies make such differences readily apparent even when the stones are visually quite similar.
Why So Difficult?
If each sapphire deposit has its own unique chemical mix, in theory it should only be a matter of careful analysis to figure out where a stone came from. But there remains an element of art to determining country of origin, partially because of the limitations of the science itself, and partially because no one has yet compiled complete data on each gem locality.
First, although each chemical mix may be unique, some elements will overlap. For example, iron is always found in blue sapphire, because that is the key ingredient that gives the stone its blue color. Those crossovers will be more extensive in deposits that come from similar geological formations: For example, sapphires from basaltic deposits such as those in Thailand, Australia, and Cambodia – will share many more characteristics with each other than with sapphires from Myanmar, which formed in a metamorphic deposit.
Such crossover may make it impossible to narrow an origin down to a single country. As a result, many laboratories will report a stone as “probably” coming from one locality, but note that other origins are also possible.
Secondly, the characteristics that are unique to a given locality won’t necessarily be found in each and every stone from that deposit. The chemical mix sapphires grow in is not homogeneous, so stones can show different characteristics when they are from different areas of the same deposit.
Heat treatment can also destroy or alter evidence of origin. For example, the distinctive color zoning seen in sapphires from Andranondambo, Madagascar, becomes significantly less distinct when the stones are heated. Heat treatment can also alter silk, making it less visible.
Perhaps the greatest limitation on country of origin determination, however, is in the data available to the gemologist analyzing the stone.
“You have to have a reliable set of samples to draw information from,” says Beesley. For example, if a sapphire from Cambodia is represented as coming from Myanmar, a lab could add characteristics typical of Cambodian sapphires to its data on Myanmar sapphires. The result would be misidentification of both Cambodian and Myanmar sapphires.
This problem arises most frequently when a new deposit is discovered, says Beesley. “The most vulnerable part [of this system] is when an area is first online, because you’re flying on old information,” he says. “With no previous history and no data to access, that’s when you’re most vulnerable [to making errors].”
Despite the difficulties, labs such as AGL, SSEF, and the American Gem Trade Association Gemological Testing Center say they can issue country of origin opinions on the majority of the sapphires they are asked to evaluate.
“We express an opinion, our expert opinion, and not just a guess after a quick glance at a stone,” says Hanni . “We issue probably 80 percent positive attributions, but it must be clear we are not speaking of basaltic deposits, as in Rwanda, Australia, Cambodia, Thailand, Shandong [China], etc., where it is of little commercial interest to know the locality. We are speaking of Kashmir, Burma, and Ceylon/ Madagascar / Tunduru.”
Because determining country of origin for a stone relies more on expert opinion than on conclusive evidence, labs will differ occasionally. They will also sometimes find it necessary to issue a “tentative” opinion, or no opinion at all.
For that reason, some labs do not offer origin reports on any stone. The Gemological Institute of America is one, and Professional Gem Sciences is another.
There are too many ambiguities involved, says Tashey, especially with stones altered by heat treatment. “We did [country of origin determination] for a while, but if you’re wrong 10, 15, 20 percent of the time, what good are you doing for the trade?”
Until either identification technology or data on gem characteristics improves, definitive answers remain out of reach.
Okay, you know about microscopes and even refractometers, but what the heck is a spectrophotometer? Here’s a quick rundown on the technologies mentioned in the article:
A spectrophotometer basically takes light and separates it into its component colors, each of which has a different wavelength. By measuring the intensity of each color, the machine creates a graph, or spectrum, that acts like a signature for the material being analyzed. Since every element has its own signature, a graph of light passing through or reflected off a gemstone will reveal what elements are in that stone.
Energy-Dispersive X-Ray Fluorescence (EDXRF)
This technique is similar to spectrophotometry, except that instead of light, an X-ray beam is aimed at the material. When the X-rays bounce off the surface of the gem, it produces Xrays with different energies, which can then be graphed like a spectrum. – SW