Building on what Miss M has said ..........
ALL pearls (including Tahitian) can be subject to dyeing, irradiation and enhancing the luster. Real Tahitian Pearls that are naturally black without any form of treatment are very expensive. My mantra, like with most jewellery buying is if the price is too good to be true, assume some form of treatment.
Here's some info that you may find interesting:-
There are three main pearl treatments that low quality pearls are subjected to 1) bleaching / artificially stained / dyeing, 2) irradiation, 3) luster enhancements. Dyeing includes the use of silver nitrate on the pearl, in order to darken the pearl’s nacre. Irradiation treatments use gamma rays to darken the pearls.
Akoya pearls have their nucleus darkened by the gamma rays, while freshwater pearls have their nacre darkened. Luster treatments are typically a clear coating placed on the surface of the pearl in order to artificially increase the luster of the otherwise dull looking pearl.
The use of silver nitrate has been used for many years to enhance pearls. This chemical penetrates the pearl, has a chemical reaction to either sunlight or hydrogen sulfide gas, and in turn can create a rich black colored pearl. Other colors can be created as well using this method, along with both organic and inorganic dyes. With the freshwater market came increased opportunities for pearl processors to experiment with these methods. The reduced cost of freshwater pearls created the perfect scenario for trying out new treatments. As for Akoya Pearls, they can be subjected to this process in order to create a much sought after "Rose", overtone. Almost all pearls are being treated one way or another today.
Most Tahitian Cultured pearls are not treated the notable exception are “Chocolate” colored pearls. The method is also used for Tahitian cultured pearls, where light colored pearls of a low color range are dyed black (originally, white South Sea cultured pearls from other countries may have been used for the same purpose). Artificially dyed pearls are found in a variety of sizes and shapes, and they are above all seen in large, so-called circle shapes.
Freshwater Pearls come in white and pastel colors from pink to purple all other colors should be considered dyed. In 1997, artificially dyed black Chinese freshwater cultured pearls in round shapes and sizes of up to 11 mm were offered on the market in Hong Kong. Since then, large quantities have reached the international market, where they can be purchased at reasonable to low prices. They have an astonishing resemblance to Tahitian cultured pearls, as they almost perfectly replicate the green to purple overtones and are already called “peacock pearls” in the Asian pearl trade. Several companies in Hong Kong offer the treatment and are often speaking of “laser treatment”. However, this is not true and neither is irradiation used. Although the exact method has not yet been made known, it is probably correct to assume that silver salts and other coloring agents are used.
Detecting color change treatments can be tricky. But here a few methods one can use:
1) one can examine the matching of the strand of pearls. A strand of natural colored pearls will have slight color variations between each pearl. A perfectly consistent strand of pearls can be evidence of color treatments. Perfect pearls are not natural.
2) One can peer down the drill hole of a pearl and look for contradictions of color. If one can see a darkened nucleus when peering through the hole, this could be evidence of iridescent treatments.
3) Sharp colors are another hint of an enhancement. It is common to dye poor quality Peach colored pearls “Pink”. Keep in mind that natural pearl colours are pastel and have a soft look to them. Bold sharp colors are normally enhanced.
Irradiation:
The irradiation method of treating pearls has different effects on Freshwater and Saltwater Pearls. For freshwater pearls, the nacre, rather than the nucleus (since freshwater pearls are mostly pure Nacre), is affected by the gamma rays. They in turn can become very dark black or brown from this treatment. The freshwater pearls are also known to develop a metallic sheen and/or a highly sought after iridescent orient surface from this treatment.
For Saltwater Pearls, the gamma rays have little affect on the Nacre however it has a huge affect on the nucleus of the pearl since the nucleus is source from Freshwater Mussels. The gamma rays darken the nucleus of the pearl, making saltwater pearls appear blue or gray. This color filters through the nacre to the surface to give the pearl a shimmering affect.
Luster Enhancement:
Almost all pearls are treated for luster in one way or another. Mostly they are simply polished in a polisher (vibrating drum) using salt or sand or even bamboo. This is done only for a few minutes at a time as not to damage the surface of the pearl. This process naturally brings out the deep luster and exposes any potential desirable orient. This is completed prior to stringing into hanks
The luster treatments based on coating a pearl with a clear coating is one of the undesirable processes in the pearl industry. The coating will probably appear fine at first, but over time will then end up cracking or peeling as the pearl becomes older. This will in turn leave only the dull luster appearance underneath the coating. This is something to watch out for because dishonest pearl processors and traders have fooled consumers in the past and will continue to attempt to do so.
To detect luster treatments, one can look at a pearl through a magnification of at least 50X. A natural untreated nacre surface is somewhat scaly, while a treated surface will appear smooth and man made.
p.s. On the GemsTV website, in the enhancement section, it states that all their pearls are subject to treatment. I know that Scott went on air and was at pains to say that their Tahitian pearls were untreated BUT that's not what their enhancement page says.