a) Black pearls
At one time, if a cultured pearl was black it was certainly stained artificially, usually by immersion in a solution of silver nitrate.
Natural black pearls do occur, although more rarely than ever, while naturally-colored black cultured pearls from Tahiti and other places are increasingly common due to the use of the black-lipped pearl oyster known as thePinctada Margaritifera.
But imitation black pearls are not so common as other color imitations since the true black pearl has a color that is far from being a matte black. In fact, the natural and cultured black pearl both have a beautiful sheen. Sometimes this sheen is a greenish hue against a black background. At other times the sheen is a bluish-black color that is very difficult to adequately describe.
b) Faint Reddish Glow Characterizes Natural Black Pearl
Natural black pearls have a faint reddish glow when viewed through crossed filters, such as when bathed in blue light from a copper sulphate solution and examined through a red filter. Artificially-stained black pearls do not have this red glow reaction.
It has been found that overlong exposure to X rays will blacken some freshwater pearls without a nucleus. But this is not so serious as possible blackening of natural pearls.
c) Some imitations have immediate giveaway signs
Imitation pearls are found in graduated and chocker necklaces of such sizes and matching symmetry and color as to be obviously anything but natural or cultured pearls. The immediate giveaway reason is that so many large, perfectly matched pearls would cost a small fortune.
But less pretentious imitations still have a giveaway sign. That is the drill hole. It is never so precise in imitation pearls as it is in natural or cultured pearls, which are individually drilled. The mass-produced imitation pearls literally have their hallmark at the drill hole, which shows signs of unevenness and rough formations due to the lack of finish applied to the manufactured pearl. Thick “tears” of the coating may appear at the drill hole as coagulations.
d) 10x lens easily reveals imitations
Such telltale signs are quickly and easily revealed when an examination of the suspected imitation pearl is made with a 10x lens.
Equally, an examination of the surface of glass beads will reveal bubbles just below the surface, or a mat pattern totally unlike the wandering serrated cloissons seen in natural pearls.
Likely candidates for treated, imitation black pearls are the hard-to-sell, less popular colors. The typical treatment uses a solution containing a silver salt, such as silver nitrate. The pearl oyster picks up a silver precipitate, which is blackened by exposure to light or hydrogen sulfide gas. The treated color is non-fading.
e) Ways of detecting treated color pearls
The treated color can be detected by a number of tests, such as certain types of X-ray analysis and infrared photography. Such tests detect the treated color’s comparative lack of ultraviolet fluorescence and its response to dilute acids.
f) Radiation used to alter cultured pearls’ colors
In recent years, gamma rays (and other forms of radiation) have been fired at cultured pearls to alter their colors. The radiation is thought to have a “charring” effect on the conchiolin layer, which is the organic matter layed out in thin sheets on the external side of the oyster shell.
The new color is dark, generally a shade of gray or bluish gray, but not really like the black achieved by the silver salt treatment. The color is stable and the treatment does not make the pearl radioactive.
Naturally colored black or dark pearls usually show reddish fluorescence under long wave ultraviolet. Gamma treated colors usually show only the yellowish fluorescence typical of bead-nucleated and freshwater pearls.
Some cultured pearls derive their color from bead nuclei that were dyed before the grafting process. On rare occasions, pearls have also been found with thin plastic coatings to add superficial color. The coating makes them feel strange and, with wear, tends to develop “bald spots”.