November/December 2002 • Northern Exposure • Opal Miners Celebrate Native Deal • Golden Pearl Anxiety Fades, Questions Linger • News & Updates • In This Issue

Unnatural Blondes
Expanded Coverage on News Story: Golden Pearl Anxiety Fades, Questions Linger
By Suzanne Wade

Pearl photo courtesy Golay.

Like 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.