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
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:
- 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
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,
|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.