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How do oysters make pearls?

Pearls have fascinated people since ancient times. Their colour, iridescence, shape and smoothness make them fascinating. But do you know that more than 99% of pearls today are not made naturally?

How an oyster makes a pearl

Natural pearls are made by the pearl oyster (Pinctada radiata) from a substance called mother-of-pearl.

When a pearl oyster is injured, or when it is attacked by a parasite, it will form a ‘pearl sac’ to contain the wound. Into this wound, the oyster secretes two proteins called conchin and perlucin. These proteins together form a matrix called conchiolin, which contains many porous spaces, somewhat like a sponge. Into these spaces, the oyster secretes aragonite crystals. These are crystals made of calcium carbonate in the shape of tiny hexagonal plates (platelets). Together, conchiolin and aragonite make up mother-of-pearl. When the mother-of-pearl has formed several layers around the original object, it becomes a mature pearl.

Natural and cultured pearls

Natural pearls are extremely rare in nature. Hundreds of oysters must be checked before a pearl is found. Most pearls today are made by culturing. For this, pearl oysters or freshwater mussels reared in large numbers in artificial lagoons. A small wound is made and a tiny object (usually a piece of tissue) is inserted into it. The oyster forms a pearl sac as part of the healing process, and in two to three months a pearl will be formed. The modern culturing of pearls was started by Kokichi Mikimoto in 1916.

If you touch a pearl, it is smooth. But when you bite it, it tastes gritty. The smoothness is because of the proteins, the gritty taste because of the aragonite.

It is said that Queen Cleopatra (69-30 BC) offered the Roman general Mark Antony the most expensive meal in history – a pearl dissolved in vinegar. (Since pearls are made of calcium carbonate, the acid in the vinegar dissolves them.) It apparently cost ten million sestertii (the Roman currency), which comes to about USD 15 million today!

How artificial pearls are made

Artificial pearls are made by a very different process. Usually a glass bead is coated with a substance called essence d’orient. Essence d’orient is nothing but fish scales, dissolved in an organic solvent. It is the same thing that adds the glossy or pearly finish to your lipstick or nail polish! So why not make your own pearls by applying three or four coats of nail polish on a few glass beads?

An artificial pearl will not taste gritty, that’s how you can make it out from a real pearl.

What makes pearls shine

The peculiar shine of pearls is called pearlescence. This is a form of iridescence, in which the colour of the pearl changes according to the angle from which it is seen. The pearlescence is caused by the platelets of aragonite in the mother-of-pearl, which are all arranged at different angles to each other.

Today, ‘pearlescent paints’ are available based on the same principle, so you can paint your cars or houses to have different colours seen from different angles.

Some fun facts about pearls

When a pearl is weighed, its weight is expressed in a unit called the momme (pronounced mom-mi). A pearl of one momme weighs 3.75 grams. As a pearl weighing 1 momme costs about USD 187.5, the pearl Cleopatra drank would be about 80,000 mommes!


Tags :     Everyday Chemistry     Calcium carbonate     pearls     conchiolin     cultured pearls     momme     aragonite    

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