Fraud with cultured pearls can be detected

Tag : coloration material

At the beginning of the 20th century, the Japanese managed toproduce the first round cultured pearls. Pearl culture entailsinserting small round pellets, usually made of oyster shell nacre,into the oysters. The important factor during this operation isthat a small section of the mother-of-pearl forming tissue mustalso be implanted. The oyster then gradually coats the foreign body- the core - that has been introduced with many layers of nacre. Inthis way, a new pearl is produced over a period of two to threeyears. Cultured pearls can be produced in all regions of the worldwhere natural pearls form. Most pearls on the market today arecultured pearls, as real pearls are rare - and expensive.

In China, sweet-water pearl farming now produces pearls for themass market rapidly and with relative ease. "The oysters areeasy to care for and the pearls grow very quickly and can becultivated from a core or even from very small tissuesamples," explains Ursula Wehrmeister. The gemmologist notesthat Chinese farmers implant up to 60 cores into one oyster - aform of biological mass production. At the same time, Chinese pearlculture does produce some oddities: Large discs or hemispheres areimplanted to obtain custom-made pearls for rings and ear-rings -even Mickey Mouse has served as a model. "Traditional Japanesesweet-water pearl farmers, on the other hand, implant only one ortwo cores, but are rewarded for their troubles with a very goodquality", states Wehrmeister.

High production volumes, and possibly also the manner in which theoysters are cultivated, means that most of the Chinese yieldconsists of rejects that cannot be used. "We suspect that theanimals are subjected to enormous stress," claims Wehrmeister,thus explaining one possible cause of this poor quality. The pearlsthey produce are not round, but misshapen and cannot be processedby the jewellery industry. The problem of vaterite is also becomingmore common.

Mother-of-pearl, and thus also pearls, are biominerals consistingof calcium carbonate with a small proportion of organic substances,rather like a brick wall made from bricks and mortar. Inparticularly attractive, lustrous pearls, the inorganic calciumcarbonate fraction is mainly made up of aragonite. The scientistsfrom Mainz found that Chinese pearls contain more vaterite, notonly within the pearls, but also on the outside, where thesubstance forms a matt surface with white spots, rendering thepearls unsuitable for sale.

High-quality Chinese pearls, however, cannot be distinguished fromJapanese pearls with the naked eye, even by experts. "We canuse trace element analysis to establish the origin of the pearlsbeyond doubt," explains Dorrit Jacob, a geochemist. A UV laseris used to cut an almost invisibly small sample in a size rangingof 5 - 100 micrometres from the study material (by way ofcomparison, a human hair has a diameter of about 40 micrometres).This mini-sample is rinsed into the analysis device with the aid ofan inert gas and the content of trace elements, particularly bariumand strontium, can then be established.

The ratio between barium and strontium in comparison with totalstrontium content indicates the origin of the material. "Wehave been developing this practically non-destructive test methodsince 2006 in order to distinguish between Japanese and Chinesepearls," says Jacob. "We are also able to use thismethod, known as laser ablation ICP mass spectrometry, to determinewhether pearls contain vaterite and whether certain sapphires havebeen subsequently treated." The large number of especiallyorange- and blue-colored sapphires currently on the market cannotall be natural. This means that sapphires with a less markedcoloration, which would normally not be marketable, have beencolored more brightly with beryllium.

The next step for the two scientists will be to analyze coral inmore detail and to create a basis for learning more about thestructure and origin of this resource created by undersea creaturesand the jewellery that can be made from it.