Monday, July 23, 2012

Uncultured pearls

Pteria penguin oyster shell with a small blister pearl. Blister (left) and eleven pearls from this oyster species are displayed in the foreground.

M.S. Krzemnicki, SSEF Swiss Gemological Institute
Natural Pearls: The beauty of diversity
Pearls have bacome beloved and affordable jewels since the emergence of large-scale pearl farming. Especially the huge production of cultured freshwater pearls from China has rendered necklaces containing such pearls accessible. Efforts to considerably improve culturing practices have resulted in ever better and more homogeneous pearls available for the world market. Unfortunately, these developments have overshadowed much of the exclusivity and singularity of natural pearls and relegated their particularities to a state of neat obscurity. Natural pearls are just not always perfect. It is specifically this individuality that makes natural pearls unique jewels.

In this and the following articles, the focus will be on natural pearls: beginning with pearls that present a mother-of-pearl lustre, to later examining porcelan-like natural pearls from mollusks (part 2) and to finnaly discussing those originating from marine gastropods (part 3).

Salty or sweet:

Numerous rivers in central Europe were more or less well known for their pearls in past times. The pollution of rivers due to industrial waste, means that these pearl-forming Mollusks have become rare and are protected species - reserving the presence of freshwater natural pearls to antique jewellery, these days. The number of natural saltwater pearls is much greater, being found from the Persian Golf to the Indian coast and all the way to the Pacific. The difference a criterion.

Freshwater pearls (both natural and cultured) have much greater concentrations of Manganese than saltwater pearls. Pearls are analysed using X-rays at the SSEF, both to visualise the internal structure of the pearl (radiography) and to record the X-ray luminecences of the pearls (freshwater pearls display a bright reaction due to their more elevated Mn concentration, whereas saltwater pearls remain dark). But it often remains difficult to classify physical or visual (e.g. colour) characteristics, which might facilitate classification. We have been able to analyse an important number of pearls in our laboratory in the past year with which Thomas Hochstrasser (Dörflingen) also kindly lent the corresponding shells for our research on natural pearls.

Pinctada maxima:

This especially large oyster species produces up to 20-30cm mighty shells, which often display beautiful mother-of-pearl. One differentiates between the Silver-lipped and Golden-lipped Pearl Oyster, depending on the colour of the mother-of-pearl towards the rim of the shell. Pinctada maxima can be found in the East Indian Ocean all the way to the tropical west Pacific; especially in Burma, southern Thailand, Indonesia, Philippines and right up to Australian coast. In practically all these areas, pearls are cultured in Pinctada maxima host oysters and are marketed as the widely desired South Sea cultured pearls. It is often forgotten that natural pearls can also grow in these pearl oysters.

In this case, an irritation such as the snapping of a crab or the drilling of a borer is often the trigger for the formation of a natural pearl. Depending on wether the pearl lies completely in the organism, or adheres to the shell, or merily of a nacre-covered mount on the shell; pearl, a blister pearl or a blister. Pinctada maxima natural pearls generally have a colour ranging from white to cream or gold/yellow. Unlike most South Sea cultured pearls, which have a round nucleus, these natural pearls are commonly individual formed, ranging from a slight teardrop shape all the way througt to barock.

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