What makes a pearl... A PEARL

PEARLS ARE THE RESULT OF SELF DEFENSE BY A MOLLUSC

Pearls of different types

This reaction starts when the mollusc covers an intruder with epithelial cells which form a pearl sac around the offending object, these cells in turn deposit concentric layers of nacre around the intruder and layer by layer form a pearl. If the mollusc does not react in this way it will die.

NACRE

Nacre layers in a pearl

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

Epithelial cells produce nacre as mother or pearl on a shell

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.

LUSTRE

lustre reflection and refraction on nacre

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.

PINTADINES

Pearls from the Pintadine family

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

Freshwater pearls from the Unionide family

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.


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