NecklacesIn 1920, Rosenthal suggested a generally basic technique for distinguishing dyed pearls: “When a [natural] blaclz pearl is scratched, the powder is white, however on account of a misleadingly shaded pearl, the powder is black.” While this strategy is precise for some dyed pearls, it is likewise damaging. Additionally fruitful trials in the 1970s with a Vickers hardness machine-(komatsu and Alzamatsu, 1978) were never adjusted for routine testing on account of their damaging nature. Today, in view of the modern gear and experience expected to recognize treatment, conclusive pearl testing requires the assets of a very much prepared gemological or pearltesting research center.
Presently gemologists at most Western gemological research facilities regularly utilize long-wave U.V fluorescence, X-ray fluorescence, X-radiographs, visual perception, and minuscule examination to independent treated from natural-shading pearls. These tests are frequently utilized as a part of conjunction with different strategies to decide first whether the pearl is natural or cultured (figure 20). Our essential worry in the accompanying dialog, notwithstanding, is their application in isolating natural-shading from treated blaclz pearls.
U.V fluorescence can be analytic in this detachment. For instance, the fluorescence of natural-shading blaclz pearls regularly shifts from brilliant red (pearls from Baja California) to dull ruddy cocoa (Tahiti pearls) when presented to long-wave U.V radiation, while dyed pearls are typically latent or fluoresce a dull green (Benson, 1960; Crowningshield, 1970; Fryer et al., 1987; R. Crowningshield, pers. comm., 1989).
X-ray fluorescence ghastly investigation includes presenting the example to X-rays and measuring the wavelengths discharged with a spectrometer (Komatsu and Alzamatsu, 1978). Silver utilized as a part of the different silver salt medicines will be identified. This test must be performed carefully, by an accomplished expert, since the specimen pearls have been known not cocoa black if presented to the x-rays in a wave-length dispersive x-ray fluorescence unit. Thusly, most diamond testing labs utilize a vitality disersive XRF unit (as portrayed in Liddicoat, 1987).
X-radiography, a system created in the 1920s to particular natural from cultured pearls, has been talked about in subtle element in the standard gemology writings (see, e.g., Webster, 1983; Liddicoat, 1987). The strategy chips away at the rule that diverse materials, for example, nacre, conchiolin, and silver shift in their level of straightforwardness to X-radiation (Brown, 1979). Cultured pearls for the most part show high help between the core, with its dull ring of conchiolin, and the nacre. In a silver-treated pearl, the silver tends to amass in the range of the conchiolin. Since silver is hazy to X-rays, this territory ordinarily seems white on the X-radiograph.
The white ring or territory around the core of a treated blaclz pearl is now and again called an inversion ring (Fryer et al., 1986, pp. 173 and 174). Additionally utilized as a part of pearl recognizable proof is visual perception supplemented by tiny perception. To the unaided eye, dyed materials will some of the time demonstrate a curiously even conveyance of shading all through the pearl and now and again all through a whole strand of pearls. With amplification, dye can be seen concentrated around the drill opening and notwithstanding stretching out into the pearl in a vein-like design (as represented in Fryer et al., 1984, p. 230).
In a few cases, dye can be recognized by utilizing a cotton swab plunged as a part of a powerless (e.g., 2%) arrangement of nitric corrosive on a subtle territory of the pearl. This is, be that as it may, another ruinous test (Fryer et al., 1984, p. 229). Visual examination is additionally helpful in distinguishing illumination in the littler Alzoya pearls. Light will obscure freshwater pearls and shells, yet not saltwater pearls. In this manner, since cultured pearls use freshwater-shell cores only, the dull bead core of a lighted pearl is effectively seen through the drill opening of the pearl.
The shade of the nacre is unaltered (Fryer et al., 1986, p. 173, and 1988, p. 244, figure 11). One must be careful, be that as it may, to make a few tests on every pearl, subsequent to while illumination changes the shade of the bead, it doesn’t change the response of the pearl to long-wave U.V. radiation (Matsuda and Miyoshi, 1988). Indeed, even undrilled pearls can give pieces of information to the accomplished pearl researcher by ideals of their visual appearance. The exceptional shading and metallic luster
have been refered to as signs of treatment (see, e.g., Fryer et al., 1985).
It ought to be noticed that size can be another essential marker of treatment. Little, adjusted pearls 8 mm or less with extremely blaclz, gray, green, or blue-green tones are by and large treated pearls – that is, littler Alzoya pearls that have been dyed or illuminated following the pearls developed in P. marguritifera are at times littler than 9 mm. Once more, however, it is critical to test pearls carefully, on the grounds that a few territories, for example, Baja California create a radiant blaclz pearl of natural shading in little sizes. Black iilzeshi” pearls can likewise happen in little sizes.
As of late, Japanese researchers have driven the route in pearl research. In 1978, Komatsu and Alzamatsu reported the achievement of their trials with infrared film to particular natural-shading from dyed blaclz pearls. Alzamatsu (pers. comm., 1989) reports that the Milzimoto Research Laboratory now observes spectrometry to be a much more productive strategy to make the partition between natural-shading and silver-treated blacks. Pearls from Pinctada margaritifera have a particular retention at 700 nm that is brought on by the blaclz shade and can be effectively measured by a spectrophotometer.
The group of T Miyoshi, Y. Matsuda, and H. Komatsu has concentrated on techniques for deciding the parentage of cultured pearls utilizing laser-incited fluorescence measured by a spectrofluorophotometer. In their 1987 paper, they
reported their perception of two fluorescence crests at 450 nm and 620 nm-in l? margarjtjfera. The 620 crest was not saw in P. maxima or P. fucata martensi, on the grounds that the shells of these clams don’t contain porphyrins. This
makes it conceivable to isolated l? margaritifera pearls from those created by these two subgroups. Research, for example, this will give the premise to more conclusive pearl testing later on.
Articles source: POLYNESIAN BLACK PEARLS, By Marisa Goebel and Dona Mary Dirlam