IN today’s marketplace, many gemstones undergo some type of treatment to improve their colour or appearance and increase their value. Whilst pearls were traditionally considered unique among gems for not requiring such cosmetic work to reveal their beauty, modern advances in treatment technologies has resulted in new ways to provide pearls with a more pleasing colour, or to create artificial colours that are suitable for fashion and design purposes. Pearl treatments may also be used to hide surface imperfections—comparable to the way that cosmetics many help veil a person’s skin blemishes. Other treatments improve lustre, add weight, or chemically bleach away unwanted colouration. New treatment techniques are always being developed, often borrowing technologies from other fields, such as medicine and optics.
Today pearls have become a favourite object for enhancement. Their porosity allows them to accept many treatments more readily than non-porous gemstones, while their softness permits them to be shaped fairly easily with even the most basic of tools, such as knives and files.
The prevalence of such treatments reminds us that all gem treatments must be disclosed at every step of the supply chain. Yet within the pearl industry, there are those who want to make a quick profit at the expense of others by failing to reveal treatment information. Non-disclosed pearl treatments can have a devastating impact on the industry. This chapter will cover the various pearl treatments and offer some insights into how they may be detected.
Treatments
Dyeing
Of all the treatments applied to pearls, dyeing by immersion in a coloured liquid is without a doubt the oldest and easiest method due to their porous nature (this may well have been how the first gems were treated). Dyes of various colours are used to produce very realistic hues such as yellow or grey, as well as unnatural colours such as bright pink or electric blue. A quick visit to a jewellery exhibition or a pearl dealer will reveal the vast range of dyed pearls available in the market.
It is important to recognise that while pearls can occur in virtually every colour, some colours are less likely to occur naturally. Most dyed pearls pose no identification challenges since their bright, unnatural colours in large quantities warn consumers of their treated condition. Some dyes are less obvious, however, and even the best-equipped gemmological laboratories may find them challenging to detect. The treatments may be so convincing that even advanced testing equipment may not be able to resolve the true nature of the pearl’s colour. One of the most notable examples are ‘golden’ yellow pearls, which, if they occur naturally, are quite valuable colours.
One of the most common dyeing treatments, especially for akoya cultured pearls, is ‘pinking,’ a process in which a white pearl is given a slight pink colouration. Pink is a common colour in both nacreous (e.g., pale pink akoyas) and non-nacreous (conch) pearls. The pink dye is applied to drilled pearls to give them a more desirable pinkish tint, rather than completely changing the colour to pink. Careful examination of the drill-holes and the relatively thin layers of nacre usually reveal the pink colour concentrations typical of this treatment.
It is often more difficult to identify treatment in loose pearls than in jewellery with multiple pearls. In the latter, dyed pearls are likely to exhibit a very consistent external colouration; in the case of a strand, the dye may be readily apparent on the necklace’s thread or, again, in the drill-holes. If the necklace is very tightly strung, however, this visual evidence will not be readily accessible and the thread might have to be cut so the pearls can be examined individually.
Other dyeing methods include inserting dyed bead nuclei into molluscs, resulting in pearls with less saturated colours as seen through the translucent nacre overgrowth. In some cases, metallic oxides are injected into the mollusc’s pearl sac(s) during formation.
Brown, grey, dark grey, and black pearls that are available to a much greater extent in the market in recent years may also be the result of a treatment that radically changes their appearance. This involves the use of light-sensitive silver-salt solutions, such as silver nitrate, which can turn virtually all pearls black, or darken pearls that are light brown or grey. After soaking into the pearl’s sub-surface structure, the silver reacts to light and oxidises to a black colour. Treatment with such solutions is easy to identify by chemical analysis with EDXRF, which can detect traces of silver. This metallic silver residue also manifests itself as white areas on a microradiograph.
Heating
Heat treatment provides another option for colour alteration, but it is more difficult to identify visually. Whilst research continues, analysis of visible and photoluminescence spectra appear to be the most helpful. UV light may also help in this determination since the resulting reactions may vary enough to warrant further advanced testing, so it is more of an indicator than actual proof. However, the identification of some heated pearls remains a challenge.
Irradiation
The irradiation of pearls has been carried out for decades using a variety of energy sources. The most commonly used technique is exposure to gamma rays, which have a higher energy level than other sources of radiation, and produce a more even colouration in pearls.
Both freshwater and saltwater pearls can be treated with gamma rays. Freshwater pearls, whether natural or cultured, are the most likely to receive irradiation treatment, since their manganese content is more likely to produce the desired darkening effect owing to the change of the MnCO3 oxidation number. However, some studies believe such alteration, whether in freshwater or saltwater pearls, is due to protein destruction rather than from the Mn related changes as well as to colour centres created by CO2 - radicals. Hence, irradiated freshwater pearls usually appear much darker than their natural-colour counterparts, and they often exhibit a pronounced orient that is quite characteristic of the irradiation treatment.
This irradiation technique also works better in saltwater cultured pearls that have a freshwater bead nucleus than in natural or non-bead cultured saltwater pearls. The bead reacts with the gamma rays to produce an underlying darker appearance that is seen through the translucent nacre layers as a grey body colour. Silver to light grey irradiated saltwater strands are available in the market today.
Irradiated pearls can present an identification challenge. Apart from the strong orient, there is often little visual evidence except for the suspicious tone and saturation of the colour. With saltwater cultured pearls that have irradiated bead nuclei, the grey or darker colour of the bead, as seen down the drill-hole, may indicate irradiation treatment. If the pearl is naturally coloured, the colour should be distributed more evenly throughout the pearl and not be confined to the freshwater bead alone.
Coatings
Some pearls are coated with a variety of materials to hide surface imperfections or blemishes. Others may be coated to protect the surface lustre from wear and handling. In the latter cases, the treatment is openly declared, but somewhere down the supply chain—when the piece enters the estate market, for instance—the treatment may go undisclosed or unnoticed. Recently, some pearls have been treated with hard materials, such as diamond-like carbon thin films, to enhance their durability.
A loupe or microscope can often be used to see a coating covering the underlying nacreous surface. In very obvious examples, bubbles may be visible in the coating. Some coated pearls have an atypical reaction to ultraviolet light. This was the case for a number of Pinctada margaritifera pearls that were examined at GIA. Their unusual fluorescence led gemmologists to turn to the DiamondView instrument, which clearly detected an uneven surface coating. While coatings may also be used on non-nacreous pearls, this treatment is mostly seen with nacreous varieties.
Bleaching
Most pearls are bleached at some stage to remove unwanted stains and lighten darker colours. The process involves immersion in a dilute form of a bleaching agent—hydrogen peroxide is the most popular. The bleaching is sometimes combined with light, heat, or a combination of secondary processes needed to obtain the desired results. ‘Chocolate pearls’, for example, are Tahitian cultured pearls whose colours have been altered by a proprietary two-step process that involves bleaching.
With some kinds of pearls, bleaching is assumed. Most akoya cultured pearls are routinely bleached as part of their processing, and GIA identification reports for akoyas specifically mention this practice.
Working and Polishing
Historically in the Arabian Gulf area, highly skilled people (known as ‘pearl doctors’) could examine a pearl and determine whether it had the potential to be improved. This improvement took the form of gradually peeling the outer layers of pearl to remove blemished or lacklustre sections. The most talented of these individuals were said to be able to transform the pearl to reveal its ultimate beauty. Nowadays, the pearl doctors are largely gone. Pearls are routinely ground down or filed to improve their appearance and shape. Even a jeweller may work to slightly modify a pearl just prior to setting it in its final mounting.
Other techniques have been developed to enhance a pearl’s appearance. In some cases a series of flat facets have been placed over the entire surface of the pearl. Polishing is a routine procedure to enhance surface lustre, and when present is noted in the comments section of the GIA report. Another method involves exposing the pearls to a mild solvent, which alters the platy structures to give a smoother appearance to the nacre. This slight difference is enough to improve lustre. The change is quite subtle, however, and spectroscopic methods are again required to assist in its detection if and when it is possible to do so.
Filling
On rare occasions pearls are filled with foreign materials, perhaps to add weight, hide a surface cavity, or permit them to be mounted more securely. Sometimes unusual fillers like metal, shell, and forms of cement have been utilised. There are several possible indications of fillers, including unusual heft, discrepancies in a pearl’s internal structure as seen in a microradiograph, and unexpected UV fluorescence reactions.
Miscellaneous
Treatments Unlike gems such as turquoise, jadeite, lapis lazuli, or opal, pearls seldom undergo impregnation with oils or polymers. The only type of impregnation noted in pearls has been the application of wax towards the end of the production cycle, to enhance lustre. Laser treatment of pearls has been hinted at on occasion, but research from China has shown that this is actually just a misnomer for a form of dyeing that does not involve the use of lasers at all. (This article is reprinted with permission from GIA in two parts. Part two will be published in HKJM Mar’14.)
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