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Expanded Coverage on News Story: Golden
Pearl Anxiety Fades, Questions Linger
By Suzanne Wade
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| Pearl photo courtesy
Golay. |
ike most gems, pearls
have a long history of modification and "improvement" by humans. Most
in the pearl industry agree these treatments should be disclosed to buyers, but
admit that such disclosure isn't always as complete as it should be.
Pearl treatments range from the accepted practice of bleaching to the almost
universally disliked process of adding a polymer coating to improve the luster.
In colored pearls, one of the greatest concerns is the alteration of the color.
Pearl color is most commonly altered in one of three ways: irradiation, dyeing,
and heat treatment.
Irradiation gives pearls a gray color, although the exact shade depends on
the type of pearl involved. In freshwater pearls, the entire pearl turns black
to gray. In cultured saltwater pearls, it's generally only the shell bead in the
center that's affected by radiation, turning a black that shows through the nacre
and causes the entire pearl to appear dark gray or black. These treatments are
usually detected by ultraviolet (UV) spectroscopy, or by microscopic examination
of the drill hole, which will reveal the disparity between the surface nacre and
the shell bead.
Dyeing can produce just about any color pearls come in, including golden. Depending
on the dye used, different shades are produced. For example, silver nitrate has
long been used to stain pearls a silvery-black. In goldens, organic dyes are used
most commonly. "The idea is to replicate a color appearance that is desirable
and also overlaps with the best colors that are produced naturally," says
Tom Moses of the GIA Gem Trade Lab in New York. "There are a number of agents
that can be used to make this coloration, but it's still a limited range."
Dyeing is typically applied after drilling, and frequently leaves color concentrations
in cracks, fissures, and drill holes. These dye residues can be difficult to see,
particularly in strung pearls, so detection usually requires careful visual examination
with magnification.
Heat treatment is the newest kid on the block, and is currently only known
for golden pearls. Treatment methods are typically considered proprietary, so
how the color alteration is achieved remains largely a mystery. It is believed
to be stable, however, and reportedly does not incorporate bleaches or organic
dyes.
When it first became widely known, heat treatment produced a furor in the pearl
industry because it is so difficult to detect. Initially even the gem labs couldn't
be certain of their ability to identify treated golden pearls.
In the last few years, however, the labs have made significant progress in
detecting heat treated pearls through UV reflectance spectroscopy. Unusual brownish-orange
fluorescence to long-wave UV radiation, color concentrations in surface defects,
and a greater concentration of color in the surface of the nacre have also been
noted in some heat treated pearls.
Such factors allow gem labs to identify a majority of heat-treated pearls,
although even the labs admit they cannot guarantee detection of all heat treated
pearls. "There are definitely occasions where we are able to write 'natural,'
and definitely occasions when we're comfortable [the color] is due to something
else, such as heat or irradiation," says Ken Scarrett, head of AGTA's lab
in New York. "But there may be situations were we aren't able to do that,
where the data just doesn't match."
In addition, identification of heated golden pearls in the field remains largely
a matter of guesswork, since most of their characteristics can only be identified
through the use of sophisticated equipment. Experienced pearl dealers suggest
the best way to identify treated pearls is by watching for telltale signs of a
too-good-to-be-true bargain.
Other dealers suggest having the seller put "untreated" on the invoice,
with a guarantee the pearls can be returned if a laboratory determines they have
been heat treated. And as ever, apply the principle of "caveat emptor"
when purchasing golden pearls.
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