pearls look like. People who work with black or yellow pearls on a regular basis can usually spot dyed pearls instantly. But even experts can be fooled. So when making a major purchase, have your pearls tested by an independent gem lab.

Magnifier Test:  Examine the surface of the pearl with a 10-power magnifier (loupe). If the color in or around the

blemishes is stronger and more intense than the rest of the pearl, this is a good sign the pearl is dyed. After dealers examine sample dyed pearls in this manner, they can easily detect dye concentrations when blemishes are present. Absence of visible dye is not proof of natural color; not all blemishes of dyed pearls show dye.

Many appraisers, jewelers and gemology students use this book as a reference, and they're interested in some of the more technical ways of detecting dyed pearls. The following methods require special equipment and are not cost effective for the consumer. Nevertheless, some lay people are interested in tests which gem labs may conduct: Infrared Test    If you have a camera, you can photograph the pearls with color infrared film. Naturally colored pearls tend to look blue, whereas pearls colored with silver salts generally look yellow (or range from greenish blue to yellow green). (Komatsu and Akamatsu, Gems & Gemology, Spring 1978)

Fiberoptic Test: If black pearls appear brownish under fiberoptic illumination but not under tungsten light-bulbs, this suggests they were dyed with silver nitrate. Good quality, natural-color Tahitian pearls usually retain their normal colors under fiberoptic lights. Occasionally, low-grade, mottled Tahitian pearls look brownish.

Fluorescence:  The pearls are examined under long-wave UV radiation. Natural-color black pearls will generally have a fluorescence ranging from a bright red (pearls from Baja California) to a dull reddish brown (Tahitian pearls). Dyed pearls tend to show no reaction or else fluoresce a dull green.

Microscope:  The pearls are viewed under a 100+-power microscope through crossed Polaroid lenses. Traces of the chemical coloring can be seen.

X-radiograph:  An x-ray photo called an x-radiograph is taken of the pearls. If the pearls are dyed with silver salts, a pale ring between the nacre and the shell bead nucleus can often be seen. In addition, there is less of a contrast between the bead and the nacre.

X-ray Fluorescence: The pearls are exposed to x-rays. Then the emitted wavelengths are measured with an instru­ent called a spectrometer to detect trace elements such as silver on the surface of the pearl. As you can see, there is a wide variety of tests for identifying dyed pearls. Use a combination of the simple tests to help you spot obvious cases of dye. But when it comes to making a major purchase, get help from professionals.