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January 2000

Gemstones & Pearls: Gemology

Origin of the Species

You can't judge corundum by its cover, says AGTA lab

Determining a gemstone's origin is a hotly debated issue in the trade, and the pressure is on for laboratories to provide this information on quality reports. Some experts believe a gem's inherent beauty, regardless of its source, is the only thing that matters. But the market favors gems that can be proven to be from a particular locality. All other things being equal, gems from recognized and desirable localities command premiums.

To make an origin determination, a lab needs expertise and instrumentation. Deep blue sapphires with a violet tinge and sleepy, hazy appearance were once a sure indicator of Kashmir origin. Then a few sapphires from the Madagascar islands in the Indian Ocean appeared with these same qualities. Other Madagascar sapphires look different and may be confused with Sri Lankan or African gems. "The variety of sapphires Madagascar produces is amazing," says Ken Scarratt, director of the American Gem Trade Association Laboratory in New York City. Scarratt spoke about the determination of origin in corundum at an invitation-only gathering in New York City in November.

More than Appearance

While outward appearance is important, the lab considers many other factors and uses multiple tools before making a call on origin.

A basic understanding of where sapphire, or corundum, is mined and the types of deposit found in each area is the first step for the AGTA lab scientists and technicians. For example, corundum is mined in basalt, marble or metasomatic deposits, each of which endows the material with certain unique characteristics.

Scarratt says the AGTA lab concentrates on three major clues in the gemstones themselves:

  • Inclusions.
  • Trace element chemistry.
  • Light absorbency characteristics.

Determinations are made only when at least two of the three categories have been positively answered.

Microscopic evaluation reveals tell-tale inclusions that indicate a sapphire could come from a distinct locality. "Thai sapphires tend to have soft-looking silk," Scarratt says, "whereas Madagascar sapphires have rounded crystal inclusions and whitish color zoning. Kashmir sapphires may contain tourmaline inclusions or dust-like streamers."

Beyond the Limits

When traditional microscopy reaches its limits, more sophisticated machinery is put to use. "Micro-Raman spectroscopy can identify inclusions as small as one micron," he says. For example, the lab recently identified small apatite crystals in a ruby.

Scarratt also checks energy-dispersive X-ray fluorescence because different gems give off characteristic X-rays. These often help identify coloring agents such as chromium, iron, vanadium and titanium. The lab's computer-aided technology can calculate the exact percentage of each. Vietnamese rubies traditionally have high chromium and vanadium content and low iron content, for example, while Thai rubies have equal amounts of iron and chromium.

How a gem absorbs and emits light can be characteristic of a given locality.

The technology for synthesis and treatments has changed the nature of identification. The AGTA lab came across a heat-treated Ramaura synthetic ruby that was hard to identify as laboratory-grown, though ultimately instrumentation allowed it to be identified. Scarratt underscores the AGTA lab's need to receive samples for study on a continued basis to enlarge its understanding and database.
"Determination of a gem's origin is not as simple as looking at a stone and saying it must be from 'X' locality," he says. "Looking at a gem is only the first step in a long ride to a conclusion."

by Robert Weldon, G.G.

Just looking at a sapphire or ruby does not provide enough information to support a determination-of-origin report. Corundum suite is courtesy of Radiance International, San Diego, CA.
Inclusions in corundum – such as this iridescent liquid inclusion – and color zoning provide a laboratory with such clues as whether it's natural, whether it's treated and where it originated. If magnification doesn't provide diagnostic results, other high-tech instruments can be used. These inclusions were observed in a sapphire from Pailin, Cambodia, at 40X.



Copyright © 1999 by Bond Communications.



 

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