Pearls.

In common chemical jargon, pearl is simply aragonite, with a certain amount of protein from the albumin group, known as conchioline. Aragonite, calcium carbonate, is a common substance that we have huge mountains of, even in Poland, so where does the unusual value of the pearl come from, and how about its commercial worth? This comes down to three factors, their incredible beauty and shape, their exotic origin and of course, their rarity. Pearls are a creation of the epidermis of the mantle, which wraps around the body of the creature and creates a external calcium-conchioline skeleton, which we know as a shell. The inside of a shell is often unusually smooth and possesses attractive optical qualities. It is known as mother-of-pearl, and is used extensively for decoration. In previous times it also had industrial uses, such as in button making, but has since been substituted by various plastic materials. 

The colouring and properites of animals usually have some adaptive quality behind them as a result of evolution. In the case of the unusual properties of the mother-of-pearl, it is different. It is hard to imagine the role of the beautiful iridescent properties of the surface of the shell, as they often live in darkness, and regardless, light is not able to penetrate the thick shell. Here we have an example of a property, which is not helpful for survival, but neither does it do ant harm, and essentially it also cheap. In one word, not everything is about adaptation. Shells, due to their hardness have played a huge role in many cultures of the world as some of the first tools used by primitive man. The mantle plays and important defensive roll, and this function is also tied up with the formation of pearls. When a foreign body enters into a mollusc and begins to irritate the mantle epidermis, the cells of the epidermis begin to excrete the same substance which the shell itself is made of.  But this is done  in such a clever way that it isolates the foreign body. This is an important matter - it could be a parasite, a sharp grain of sand, or a piece of shell that is damaging the creature. 


The isolation by building a mineral armor against the intruder is an ingenious form of protection against dangerous factors. It is therefore fair to say that a pearl is the product of suffering - of the mollusc. 
Depending on the way that the mollusc deals with the unwanted guest, we have pearls of different shapes. They can be pear shaped, flat, irregular (called barock), but most valuable are ones that are perfectly spherical. Pearls also tend to vary greatly in their size. The largest jeweler's pearl is the size of a dove egg. The largest pearl ever recorded weights 1860 grans - this is the unit used by jewelers to ascertain mass (1gran = 65 mg) - therefore about 121 grams. The commercially valuable pearls occur in the mantles of a couple of species of molluscs, but the true producer of jewelerly quality pearls is the pearl oyster (Pinctada margaritifers). It occurs in tropical seas, predominantly in the Indian Ocean, and is 'caught' by pearl divers. Such divers mastered the art of diving many of hundereds of years ago, and can easily reach depths of up to 25 metres without any equiptment. 
Pearls are usually white with a rainbow shine, but sometimes they can have a yellow or green tinge, and even be entirely black. There is no clear evidence as to what determines the colour of a pearl, but it is most likely to be due to their specific surroundings, the composition of the sea water, the sea bed itself as well as the nutrients available. For some reasons the most interest is generated by black pearls that occur in some of the atolls of the Pacific, and currently make up a large percentage of the incomes of the inhabitants of the islands. (see next item)

QZC06::036];[QIL96::013]p140-3;EB9,226;QCP05096\13\140-1;[QAB04::379]p601,8

 


 

(C) (selected from publications of 
 R. Antoszewski

Titirangi, Auckland, 
New Zeland

Site Meter
February  2003

v.16