Pearls are the product of the act of self preservation by a mollusc.
In other words the creation of a pearl is caused by the protective reaction of an oyster or mussel to the accidental or deliberate introduction of a foreign body into its organism. This reaction starts by the mollusc covering the intruder with epithelial cells which will form a pearl sac around the intruder, which in turn deposit concentric layers of nacre that surround the offending object and slowly form the pearl, layer by layer. If the mollusc does not react in this way it will die.
To understand further the origins of pearl formation lets discuss what makes a pearl... A PEARL.
Nacre is made up of calcium carbonate in the shape of tiny crystals called Aragonite. The Aragonite crystals are held together with a glue-like protein called Conchiolin.
Our teeth are also made of calcium carbonate, however the protein that holds our teeth together is stronger hence when we "test" pearls against our teeth it is not only unhygienic but totally undesirable as our teeth will scratch the pearl.
Epithelial cells produce nacre and are therefore essential to pearl formation. They are found in a special tissue called the mantle at the hinge of the mollusc. As seen in the photograph shown, nacre grows not only on pearls but also as mother of pearl on the interior of the shell. Nacre layers within the shell of the mollusc act as a protective shield against the outside world. The only difference between pearls and mother of pearl is that in a pearl the layers are concentric and in mother of pearl they are flat or straight.
To understand Lustre we need to appreciate that nacre layers play a vital role in the pearl's lustre. Nacre layers are very thin, translucent and reflect light, thus creating the pearl's distinctive lustre. Generally the thicker the nacre with regular, thin and translucent layers, the finer the lustre will be on the pearl. In other words lustre is caused by the reflection of light on the surface of the pearl and the refraction of light as it passes through the layers of nacre. This effect appears to make the pearl glow from within, see diagram above and to the right.
Seawater pearl producing shellfish are not in fact oysters. Although for ease everyone has and will continue to call pearl bearing shellfish oysters, for the most part seawater pearl bearing molluscs belong to the Pintadine family. Within the Pintadine family there are seven pearl producing shellfish; unlike their edible sedentary namesakes, the Pintadines are not edible and are mobile from one generation to another.
The mobility of the Pintadine shellfish is due to their reproductive cycle, as they can change gender from one season to another. When conditions are right one shellfish releases spermatozoa into the water which begins a chain reaction on all other pearl producing Pintadines in the area. They release eggs and spermatozoa into the water; these are mixed at the mercy of the currents and larvae are formed. The Larva propels itself with a small foot in the water and grows into spat. At 45 days it is ½ inch long and ready to attach itself to a suitable growing spot with plenty of light, food and warmth.
Unionides produce the majority of the freshwater pearls that we know. These are bivalve shellfish, normally referred to as mussels or mulettes; they too are mobile and mainly inedible.
The mobility of pearl producing mussels is also due to their reproductive cycle; in this case the fertilised egg enters the gills of a fish and feeds off its blood turning into larva. When the larva has been in the host fish for about two months and the fish reaches a particularly suitable stretch of water, the larva disengages from the fish and settles.
What is a cultured pearl and how are they made
The term cultured pearl implies that a technician implants into a pearl bearing mollusc an irritant which must include a graft made from epithelial cells found in the mantle tissue of a donor mollusc. The graft forms a pearl sac and within this sac the mollusc will secrete layers of nacre to cover the irritant. It is important to bear in mind that after the irritant has been introduced the process is continued solely by the metabolism of the living mollusc. The pearl farmer has no control over what these pearl bearing molluscs will produce in size, shape, colour, or even whether they will produce a pearl at all. Each pearl is truly individual.
Cultured Oyster Pearls
Begin when a technician implants into the pearl bearing oyster a nucleus made of mother of pearl or resin with an epithelial cell graft that has come from the mantle tissue of a donor oyster. The graft covers the nucleus forming a pearl sac which is rather like a placenta; this sac secretes layers of nacre to cover the nucleus that it is enveloping, layer by layer eventually forming a pearl, the nucleus turns freely within the pearl sac. Because these pearls have a nucleus as an irritant they are termed nucleated.
Cultured oyster pearls are normally harvested in the colder months when the nacre layers are thinner and hence the lustre will usually be at its best. Generally each oyster produces only one pearl, which means the oysters are looked after very carefully in a farm; predators like starfish are kept at bay, each oyster is checked and scrubbed at least three times a year.
The following diagram gives an idea of the success rate of oyster pearl cultivation, Out of 1000 cultured oyster hosts:
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