Identifying fake pearls is usually relatively easy. What can be difficult, however, is to detect treatments. Knowing what signs to look for can make this process easier.
One treatment that seems to be appearing with greater frequency is the coating of pearls. This is done to improve lustre, change colour, and/or make low-grade pearls look as if they are of high quality. Figure 7 shows part of a coated pearl strand that was sold at a gem show without treatment disclosure. The coating was easy to detect with magnification—it had the same glittery and grainy appearance as the surface of imitation pearls, but the blemishes and drill holes indicated these were cultured.
Some cultured freshwater pearls have a natural metallic lustre and gold or bronze colours when they are harvested. However, sometimes those colours and lustre are the result of a thin metallic coating or lacquer. This is so thin it can wear off before the freshwater pearls are even worn. The bald spots visible in Figure 8 are not light reflections, but areas where the coating has peeled off.
Akoya strands with a uniform pink body colour have usually been soaked in a pink dye. This is seldom disclosed, but can be determined by examining the pearls and their drill holes with a 10x loupe. Pink dye may be visible in the hole and in any cracks or pits (Figure 9).
Dyed cultured freshwater pearls
You don’t need a loupe to determine the cultured pearls in Figure 10 are dyed—their unnatural colours make it obvious. The best way to disclose these is to simply call them dyed cultured pearls. There is no need to use flowery terms such as ‘colour enhanced.’ These types of pearls typically have dye concentrations near the drill hole and in some pits and other blemishes.
Dyed golden South Sea cultured pearls
Beware of dyed golden South Sea cultured pearls. The dyed pearls seen in Figure 11 are easy to detect because of their concentrations of dye, but in most cases it is difficult to identify colour-treated pearls of this type, even with a microscope. High-tech equipment is required to confirm such pearls are of natural colour, so lab documents are advisable, especially if you don’t know the producer.
Many Internet vendors and some show vendors do not disclose their dyed golden pearls. For example, the strand shown in Figure 12 was advertised online as an Australian South Sea gold pearl necklace. Normally, when sellers identify their pearls as South Sea varieties, that means they are saltwater pearls from white-, gold-, or black-lip oysters. These, however, are dyed freshwater pearls. It is best to ask vendors if their pearls are saltwater or freshwater, then ask them to write the pearl type on the invoice and indicate if it is of natural colour or not. (Saltwater cultured pearls cost much more than freshwater cultured pearls.)
Dyed and/or irradiated grey pearls
If a black or dark grey cultured pearl is smaller than 8 mm (0.3 in.) in size, one should assume it is a dyed akoya or freshwater cultured pearl. It would be unprofitable to culture Tahitian pearls smaller than this, because larger bead nuclei can be used in black-lip oysters.
Many colour-treated freshwater pearls are sold as Tahitian pearls. Their shape—together with their dye concentrations and low price—are good clues they are not of natural colour. Even though there are many drop-shaped and irregular-shaped Tahitian pearls, they usually look different from freshwater pearl shapes. Irradiation can give them a bluish or greenish grey colour, and dye can make them darker.
Two common methods of darkening off-colour pearls are to soak them in a dye or in a silver nitrate solution followed by light exposure. The depth of the black to grey colour depends on the strength of the solution and the length of time they are soaked.