Pearl diagnosis
The Mineralogical Museum of the University of Hamburg produces pearl appraisals using X-ray methods (radiography, X-ray diffraction and X-ray fluorescence). X-ray examinations can be used to determine non-destructively whether
- natural or cultured pearls are present
- the pearls are cultured pearls
- they are cultured pearls without nuclei
- they are freshwater or saltwater pearls.
I. Natural pearls (oriental pearls)
X-ray shadow images (radiographs) of pearls reveal their growth structures. Natural pearls (orient pearls) have a characteristic internal structure that distinguishes them from cultured pearls.
II. pearls with cultured nucleus
X-ray diffraction images show the crystallographic structure of pearls. A cultured core has a different structure than a natural pearl, therefore a diffraction image reveals the cultured pearl. A special X-ray diffraction facility at the Mineralogical Museum Hamburg allows direct observation of the diffraction patterns (Laue diagrams) of pearls. This thus allows a fast and precise identification of cultured nuclei, such as those used for Akoya or South Sea cultured pearls.
III. Cultured pearls without nuclei
X-ray shadow images are also used for the identification of seedless cultured pearls, such as for Chinese freshwater cultured pearls. Seedless cultured pearls are largely constructed like natural pearls, but show characteristic features in their centers due to their formation.
IV. saltwater or freshwater pearls
Observation of the luminescence of pearls under X-rays allows a conclusion to be drawn about the origin of pearls from saltwater or freshwater environments, since only freshwater pearls fluoresce in X-rays.
Costs and contact
The examination costs depend on the expenditure and are to be inquired with Mr. Prof. Dr. Jochen Schlüter.
Diagnosis of pearls by X-rays
Pearls originate from natural populations of certain marine and riverine molluscs or they are the product of human manipulation of these molluscs. Although cultured pearls have been available for nearly a hundred years, natural pearls have always retained their high value. Thus, the distinction between cultured and natural pearls has important significance in determining the value of pearls.
The first pearl culture process was developed in Japan at the beginning of the last century. In this process, adult marine mollusks are implanted with a core turned from mollusk shell together with a piece of mantle tissue from a foreign mollusk of the same species. The implanted piece of tissue is selected to carry the nacre-producing epithelial cells of a mollusk towards the cultured core. These are the cells that form the nacre of the inside of a mussel. This piece of tissue subsequently continues to grow and encloses the inserted cultured nucleus with a so-called pearl sac. If the mussel is well cared for and living conditions are optimal, the epithelial cells now secrete nacre onto the cultured nucleus, so that after a few months a nacre layer has formed on the cultured nucleus. The result after harvesting is a cultured pearl with a large cultured nucleus and a thin nacre layer.
Since the middle of the last century, a second type of cultured pearl has been developed from breeding experiments in Lake Biwa, Japan. Here, fast-growing freshwater mussels are used, in which only the said epithelial piece is inserted without a culture core. The initially only small rice-grain-shaped freshwater cultured pearls of the 1970s have now been replaced by large, round pearls of good quality.
Natural pearls and cultured pearls are often optically indistinguishable. However, the development of natural pearls and the different breeding methods of cultured pearls leave characteristic growth features inside the pearls.
A reliable and elegant tool to make these internal structures visible and thus to reveal the character of a pearl in a non-destructive way are X-ray methods, as they are performed at the Mineralogical Museum of the University of Hamburg and made available to the public.
Pearl diagnosis by X-ray methods is based on the different penetrability of X-rays through the different components of the pearl and on their interaction with the inorganic pearl substance.
The cultured core dominating the cultured pearl consists of parallel layers of aragonite and thus differs from the spherical structure of a natural pearl. This structural difference can be made visible by X-ray diffraction. Cultured pearls with a core can be directly distinguished from natural pearls in this way.
In the case of cultured freshwater pearls without nuclei, which have cultured characteristics only in the innermost part and otherwise resemble natural pearls in structure, this method is ineffective. In this case, the X-ray shadow method is helpful, in which the pearls to be examined are transilluminated. The nacre of pearls is actually nothing more than the iridescent nacre inside shells. It consists of aragonite, a calcium carbonate, in the form of tiny platelets that are bound in an oriented manner in a network of the organic horn-like substance conchyn. Because conchyn has better transmission of X-rays than aragonite, different degrees of blackening result on an X-ray film placed behind the bead. This reveals the structure of the pearl and thus its history of formation. Thus, natural pearls ideally show approximately the pattern of the annual rings of a tree slice, nucleated cultured pearls show the circular cultured nucleus, and nucleus-less cultured pearls usually have dark irregular features in the center.
Ultimately, an indication of the origin of pearls is their fluorescent behavior. Freshwater pearls glow in the dark under X-rays in a greenish-yellow light, a characteristic that does not occur in saltwater pearls.
Further information can be found in "Perlen und Perlmutt", J. Schlüter u. Chr. Rätsch, Ellert & Richter-Verlag Hamburg, 1999.
