Gem-Session 2 “Pearls”
August 26, 2011 | No Comments
Since the antiquity pearls are noticed for their natural beauty and therefore legendary. Like “Tears of the Gods that had fallen into the sea”, “Dew gathered in sea shells” and are often seen in important royal treasures.
Although the natural pearl forming process is a complicated one I will try to explain it in a brief clear overview. Natural pearls are formed by a mollusc’s biological reaction against a foreign particle trapped between its mantle and the shell. The mollusc reacts to this by coating the entity in shell material to avoid irritation. This process forms the pearl. When the mollusc’s shell is made from mother-of-pearl (‘nacre’) the mantle secretes nacre and the resulting pearls are therefore ‘nacreous’. This is the case for oyster and mussel pearls: the pearls are always made from the same material as the shell.
For Conch pearls, the “Pink pearls”, the basic principle remains the same. Although conch pearls are non-nacreous. The mantle of ‘Strombus gigas’ secretes a shell material which has the same chemical composition as nacre, but with a different structure (a porcelain-like appearance) and the pink colour is due to organic pigments.
To learn more about pearls and conch pearls on the internet, I can recommend to you this ‘Pearl book’ of CIBJO (The World Jewellery Confederation) for the nomenclature of natural pearls. Also other interesting “pearl and Conch pearl information web-sites” such as this one by Nordskip with beautiful images and also The American Museum of Natural History in New-York ( information of former exhibition dating from 2001) and lastly “All about Gems”. However, bear in mind that the latest up-dates about pearl-types, research possibilities and the newest colouring techniques and technologies are not discussed in all of the above mentioned internet-sites.
Avaïki Pearls
These sustainable cultured South Sea pearls are naturally greyish tinted, fully round, with an average diameter of between 9 and 13 millimeters, the average thickness of the layer of mother of pearl is 1.2 millimeters (which is “thick” in comparison to the minimum of 0.8mm that is required for Tahiti pearls in general).
At this moment there are just two pearl farms who harvest about 40.000 pearls a year. Conspicuous is the fact that these pearls are cultivated in Pinctada Margaritifera, but that the majority of the pearls show a natural lighter colour than the colours which were previously considered as normal to this mollusc.
The majority of the pearls coming from the Cook-Islands are being sold as “Cook Island pearls”. Pearls of the finest qualities are launched as “Avaïki Pearls”. These pearls are classified according to weight, size, shape, lustre and quality of the surface. Particularly the lustre and the quality of the surface (as smooth as possible) prescribe the quality. Cultured Avaïki pearls can be categorised as “perfect” or as A-, B-, C-quality. A unique thing is that currently all official concerned parties of the Cook-Islands are working with this formalised grading system, so that a consistent quality is guaranteed. This means that in reality only the best pearls are chosen to become “Avaïki” brand pearls.
The Cook-Islands have decided that the added values to this solid brand are sustainability, integrity and quality. So that the lagoons will stay a guaranteed healthy, vital and clean environment that is needed to guarantee pearls of this high quality. To take care that the cultivating pearl-farmers earn a decent salary the authorities have totally revised the distribution system. Subsequently the government has also taken some supporting measures such as kindly dispersing loans for pearl farmers and starting a fund for helping to aid and train the local community. A New-Zealand organisation takes care of the audits of both production and distribution, so the distributors are part of the audit-process.
By this in principle all the ingredients are present for a successful sustainable production within natural difficult circumstances. By these “circumstances” you’ve got to think of the forces of nature like typhoons that can potentially destroy a season’s production. Therefore this project is one of those inspiring initiatives of the jewellery trade.
The newest technique for natural pearl research; X-ray Computed Micro-tomography
This non-destructive research method for pearls is, due to long measurement time and costs of instrumentation (and only one pearl at a time can be viewed) only interesting for special occasions. For example if a reasonable doubt is there that a certain “natural pearl” may have a freshwater-pearl nucleus. Take in consideration that the value of pearl strongly depends on its cultured or natural origin. Besides the interest worldwide in South Sea pearls, the technical know-how in cultivating them and the difficulty to obtain the right nucleus for the growth-process of it, makes it necessary for gem labs to be able to distinguish all elements and parts of the process.
This technique gives the opportunity to look at an object under high resolution circumstances. X-ray photos are made of the pearl in slices while rotating the pearl over 180 degrees, multiple radiographic projections taken in different directions. The software of the computer adapts this information into a 3-D models, black and white image. Then two-dimensional slices can be cut and directly seen. Thereby you can move the cursor to each and any specific place, and view that specific place from different angles.
When you would like to read specific information about this subject I highly recommend to you to download, for $12, the article “X-ray Computed Microtomography applied to Pearls: Methodology, Advantages, and Limitations” at the GIA website. This article is made by S. Karampelas, J. Michel, M. Zheng-Cui, J.O. Schwarz, F. Enzmann, E. Fritsch, L.Leu and M. Krzemnicki.
Far more practical, efficient and cheaper (and also non-destructive) and therefore more generally used is the ‘Faxitron- X ray”, wherein digital X-ray images of a certain object are shown. And thus in one image a whole pearl necklace, brooch or whatever can be investigated. In a practical way, the research will start with a normal X-ray test and then only in case of a serious doubt will there be a follow-up by a micro-tomography test.
The Conch pearls, also called the Pink pearls
Conch pearls are formed in Caribbean Seashells, mostly pinkish coloured. But there also do exist other colours like brown, chocolate and cream and orange pearls. The orange pearls come from the Melo melo in Vietnam and Thailand.
In case of the conch pearl, the light interacts with the pearl’s surface in a different way. There is no nacreous effect, as the conch pearl is not composed of nacre. As said earlier in this article it has a porcellaneous appearance instead. The micro crystals are fibrous prismatic crystals arranged perpendicularly to the surface of the pearls. So the light passing from one fibre to another gives a shimmering effect known as ‘flame structure’. This flame-like pattern is reminiscent of the optical effects of moiré silk.
My book recommendations:
“The Pink Pearl”, A Natural Treasure of the Caribbean, by David Federman & Hubert Bari, Hardcover ISBN 978-88-6130-013-2 €40. The best book about Conch pearls I’ve ever read and provided with the most beautiful images. Telling and showing from traditional sailing boats up to outstanding photos of the flame structures of conch pearl and exquisite pieces of jewellery.
“Pearls”, by Hubert Bari, hardcover, $60.
“Pearls: A Natural History”, by Neil Landman and Paula Mikkelsen. The American Museum of Natural History. Hardcover, ISBN 0-8109-4495-2 $80.
Museum recommendation:
Being a great fan of the paintings of Hans Holbein the Younger (1497/8-1543) I was recently amazed to discover at the online gallery of The British Museum the fact that he actually also designed jewellery! Although none of his pieces have survived. It’s clear he liked pearls very much! So, please go directly to their Online Gallery where you will be taken by surprise as soon as you type ‘jewellery designs by Hans Holbein’ into the database!
Acknowledgement:
I would like to thank Dr. Hanco Zwaan of the Netherlans Gemmological Laboratory for his valuable input and suggestions, and for the use of the photos and microphotographs.
I hope you enjoyed this second ‘Gem-Session’! The next one is planned for later this year.
Tags: Avaïki pearls | Conch pearls | Dr. Hanco Zwaan | Leiden | Micro-tomography
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About
Margriet Sopers, FGA
Having a diploma in jewellery, FGA and a propaedeutic of the Academy of Art followed-up by interesting years as a jewellery expert at Sotheby's & Gemeentelijke Krediet Bank, as well as being a member for years of the Society of Jewellery Historians, I am glad to share with you today's world of jewellery.