The list of non-traditional treatments a cultured pearl might go through is a bit longer than the list of traditional ones, though in essence there are only four families of treatments: Maeshori, filling, clear or natural coating and coloring. In normal jewelry dictum, customers do not need to be informed of pre-treatments, processes and traditional treatments. Generally it is assumed that most of a given type of gem will go through these types of treatments and therefore it is unnecessary to reiterate something that is nearly immutable. Optimally, non-traditional treatments ought to be disclosed to the customer, but sometimes the buyer has not been informed either and so does not have the information to pass on. It is always good to be aware so that you can discern for yourself.
The first is a family of treatments reputed to be performed on most pearls; but even though they are not traditional treatments since they are relatively new, they are generally not disclosed. Instead, they might be referred to as a pre-treatment, if they are mentioned at all. These are subtle treatments that happen soon after the pearls are harvested, often in conjunction with traditional treatments. These treatments are known as “Maeshori”, which seems to be a bit of a catch-all phrase that refers to some form of heat treatment of which there are several variants. Generally each factory has its own preferred method of Maeshori, some versions of which include adding a coating to the exterior of the pearl. The goal is to tighten the nacre, often by pulling a little of the natural moisture out of the pearl. This closes or fills minute spaces giving a tighter smoother surface that reflects and refracts light better, resulting in a better luster. Depending on the skill of the technician, the nacre may become brittle and the luster may fade with time, some as quickly as just six months and progressing to dullness in a few years.
One known form of Maeshori involves soaking the pearls in solutions including mineral salts, methyl alcohol or hydrogen peroxide and methyl alcohol. They are heated while soaking for anywhere between 30 minutes and 30 days. This method can be performed in junction with the bleaching process, and therefore claimed to simply be a part of the bleaching process, which is a traditional treatment. The goal is to swell the nacre platelets to close or fill the minute spaces.
Another form simply exposes the pearls to hot dry air using equipment that resembles a fruit or nut dryer. This method dries and tightens the surface nacre, which decreases the number of minute spaces to improve luster.
Sometimes, a pearl’s nacre might not be sitting properly on the bead nucleus - there might be an air bubble between layers or the bead might be entirely loose within the nacre. In instances like this, an epoxy-like substance is inserted to stabilize the pearl and also sometimes to add weight.
Some pearls are coated with lacquer. There are several reasons for this. They may be used to stabilize a thin nacre, to protect the nacre, and to keep it from chipping. They are also used to improve luster in much the same way as wax and oils: by filling holes, crevices and cracks. The lacquer may also be dyed the same color as the pearl to camouflage surface features, even coloring, and make strands of pearls match. This lacquer wears off over time, revealing and often damaging the surface of the pearl it coated.
One of the last resort treatments a pearl will receive, and the treatment that most drastically changes the look of the original pearl, is to have its color changed entirely. There are many methods by which this is done. The least invasive, but most obvious is by lacquering a pearl with a darker or brighter color than is natural to that pearl. If a pearl is required to be lighter, it is normally left in bleach longer, often under heat to speed the process. Although changing a pearl’s color is considered non-traditional, because bleaching is a traditional method, bleaching pearls often falls under the “acceptable” purview.
Dying is one of the most common methods to change the color of a pearl. Often the pearl is dyed any shade from a rainbow of colors and is very easy to spot because of the unnatural shades. But pearls can also be dyed to more realistic colors, mimicking more expensive types of pearls. These dyes color the nacre, not the bead, so carefully looking down the drill hole can reveal the pearl’s original color.
One of the more common substances used to dye pearls is a solution of silver nitrate. This is similar to the silver salts that were used to darken photographs. When exposed to light, the solution decomposes and leaves a dark stain that dyes the pearl from brown to grey to black.
There are other means to change a pearl’s color as well. Heat or heat and pressure treatment can be used to turn white pearls golden to brown. Also common is irradiation. Gamma rays are used to deepen the color of a pearl. Interestingly, salt water and freshwater pearls react differently to gamma rays because the gamma rays affect magnesium which is only in the freshwater material. The majority of salt water pearls are nucleated with freshwater shell beads, so the pearl is darkened by the gamma rays darkening the bead nucleus. This means that only grey to silver can be achieved because of the interference of the paler nacre. Freshwater pearls are affected throughout by gamma rays, giving them much darker tones and often lending them a metallic to iridescent sheen that tends to look unnatural. Though, this iridescent sheen might come from another source: metal vapor deposition. The pearl is placed in a vacuum chamber and coated with Niobium, giving a metallic sheen similar to that of an oil slick on water.
There is nothing wrong with buying treated pearls, so long as you know what you’re getting. They are often more delicate and have a shorter life-span, so additional care needs to be taken. And the more treatment the pearls have seen, the less expensive they should be. The best course of action? Be aware of what is out there, and know what you buy or buy from someone you trust.