Wholesale Pearl Necklaces : Modern Pearl Identification

In 1920, Rosenthal recommended a relatively simple method for identifying dyed pearls: “When a [natural] blaclz pearl is scraped, the powder is white, but in the case of an artificially colored pearl, the powder is black.” While this procedure is accurate for some dyed pearls, it is also destructive. Similarly successful experiments in the 1970s with a Vickers hardness machine- (komatsu and Alzamatsu, 1978) were never adapted for routine testing because of their destructive nature. Today, because of the sophisticated equipment and experience needed to identify treatment, definitive pearl testing requires the resources of a well-equipped gemological or pearltesting laboratory. ( source : Wholesale Pearl Necklaces )

Currently gemologists at most Western gemological laboratories commonly use long-wave U.V fluorescence, X-ray fluorescence, X-radiographs, visual observation, and microscopic examination to separate treated from natural- color pearls. These tests are often used in conjunction with other procedures to determine first whether the pearl is natural or cultured (figure 20). Our primary concern in the following discussion, however, is their application in separating natural-color from treated blaclz pearls. ( source : Wholesale Pearl Necklaces )

U.V fluorescence can be diagnostic in this separation. For example, the fluorescence of natural-color blaclz pearls commonly varies from bright red (pearls from Baja California) to dull reddish brown (Tahiti pearls) when exposed to long-wave U.V radiation, while dyed pearls are usually inert or fluoresce a dull green (Benson, 1960; Crowningshield, 1970; Fryer et al., 1987; R. Crowningshield, pers. comm., 1989). ( source : Wholesale Pearl Necklaces )

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X-ray fluorescence spectral analysis involves exposing the sample to X-rays and measuring the wavelengths emitted with a spectrometer (Komatsu and Alzamatsu, 1978). Silver used in the various silver salt treatments will be detected. This test must be performed carefully, by an experienced technician, since the sample pearls have been known to turn brown-black if exposed to the x-rays in a wave-length dispersive x-ray fluorescence unit. Therefore, most gem-testing laboratories use an energy-disersive XRF unit (as described in Liddicoat, 1987). ( source : Wholesale Pearl Necklaces )

X-radiography, a technique developed in the 1920s to separate natural from cultured pearls, has been discussed in detail in the standard gemology texts (see, e.g., Webster, 1983; Liddicoat, 1987). The method works on the principle that different materials such as nacre, conchiolin, and silver vary in their degree of transparency to X-radiation (Brown, 1979). Cultured pearls usually show high relief between the nucleus, with its dark ring of conchiolin, and the nacre. In a silver-treated pearl, the silver tends to concentrate in the area of the conchiolin. Because silver is opaque to X-rays, this area commonly appears white on the X-radiograph. ( source : Wholesale Pearl Necklaces )

The white ring or area around the nucleus of a treated blaclz pearl is sometimes called a reversal ring (Fryer et al., 1986, pp. 173 and 174). Also used in pearl identification is visual observation supplemented by microscopic observation. To the unaided eye, dyed materials will sometimes show an unusually even distribution of color throughout the pearl and in some cases throughout an entire strand of pearls. With magnification, dye can be seen concentrated around the drill hole and even extending out into the pearl in a vein-like fashion (as illustrated in Fryer et al., 1984, p. 230). ( source : Wholesale Pearl Necklaces )

In some instances, dye can be detected by using a cotton swab dipped in a weak (e.g., 2%) solution of nitric acid on an inconspicuous area of the pearl. This is, however, another destructive test (Fryer et al., 1984, p. 229). Visual examination is also useful in identifying irradiation in the smaller Alzoya pearls. Irradiation will darken freshwater pearls and shells, but not saltwater pearls. Therefore, since cultured pearls use freshwater-shell nuclei almost exclusively, the dark bead nucleus of an irradiated pearl is easily seen through the drill hole of the pearl. ( source : Wholesale Pearl Necklaces )

The color of the nacre is unchanged (Fryer et al., 1986, p. 173, and 1988, p. 244, figure 11). One must be careful, however, to make several tests on each pearl, since while irradiation changes the color of the bead, it does not change the reaction of the pearl to long-wave U.V. radiation (Matsuda and Miyoshi, 1988). Even undrilled pearls can provide clues to the experienced pearl scientist by virtue of their visual appearance. The intense color and metallic luster
have been cited as indications of treatment (see, e.g., Fryer et al., 1985). ( source : Wholesale Pearl Necklaces )

It should be noted that size can be another important indicator of treatment. Small, rounded pearls 8 mm or less with very blaclz, gray, green, or blue-green hues are generally treated pearls – that is, smaller Alzoya pearls that have been dyed or irradiated-since the pearls grown in P. marguritifera are seldom smaller than 9 mm. Again, though, it is important to test pearls carefully, because some localities such as Baja California produce a lustrous blaclz pearl of natural color in small sizes. Black iilzeshi” pearls can also occur in small sizes. ( source : Wholesale Pearl Necklaces )

In recent years, Japanese scientists have led the way in pearl research. In 1978, Komatsu and Alzamatsu reported the success of their experiments with infrared film to separate natural-color from dyed blaclz pearls. Alzamatsu (pers. comm., 1989) reports that the Milzimoto Research Laboratory now finds spectrometry to be an even more efficient method to make the separation between natural-color and silver-treated blacks. Pearls from Pinctada margaritifera have a specific absorption at 700 nm that is caused by the blaclz pigment and can be easily measured by a spectrophotometer. ( source : Wholesale Pearl Necklaces )

The team of T Miyoshi, Y. Matsuda, and H. Komatsu has focused on methods for determining the parentage of cultured pearls using laser-induced fluorescence measured by a spectrofluorophotometer. In their 1987 paper, they
reported their observation of two fluorescence peaks-at 450 nm and 620 nm-in l? margarjtjfera. The 620 peak was not observed in P. maxima or P. fucata martensi, because the shells of these oysters do not contain porphyrins. This
makes it possible to separate l? margaritifera pearls from those produced by these two subgroups. Research such as this will provide the basis for more definitive pearl testing in the future. ( source : Wholesale Pearl Necklaces )

Articles source: POLYNESIAN BLACK PEARLS, By Marisa Goebel and Dona Mary Dirlam ( source : Wholesale Pearl Necklaces )
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