How oysters make pearls

Aha : Why do oysters have a pearl?

Cracking nuts is a man's job in many families. Dad has to work because children cannot break the shells of walnuts or hazelnuts. Fledglings don't have it easier either. Oystercatchers practice for up to two years before they can access shellfish meat without parental assistance. They learn to push their beak between the oysters' flaps and sever their sphincters or to pick open the shells of mussels.

To do this, oystercatchers need strong beaks. The clam shell is hard. Shellfish form solid shells of calcium carbonate as they grow to ward off predators. Precious pearls can also be made from the same material if a foreign body, such as a grain of sand, has been washed into the clamshell housing of an oyster. In a kind of wound healing program, the intruder is coated with mother-of-pearl and rendered harmless.

The mother-of-pearl of shellfish is very break-proof. "It's built like a brick wall," says Monika Fritz, an expert in biomineralization at the University of Bremen. The bricks, small platelets made of calcium carbonate only 15 micrometers in diameter and half a micrometer high, are piled up layer upon layer. The mortar in between is made up of organic substances, including sticky sugars and proteins. "In a millimeter of mother-of-pearl there are 2,000 such layers on top of each other."

You can recognize mother-of-pearl by its rainbow iridescence. It is iridescent. The many layers of calcium carbonate are each about as thick as the wavelength of visible light. Part of the incident sunlight is reflected on each layer and part is let through. The rays thrown back from the different depths then overlap: partly constructively, so that certain color tones are intensified, partly destructive, so that other areas of the color spectrum are extinguished.

The inner shells of pearl oysters, gyratory snails or abalones shimmer in different colors depending on the viewing angle. Pearls also shine due to this regular layer of mother-of-pearl. However, only a few types of shell produce beautiful, round pearls. Nowadays the stones are grown by inserting foreign bodies into the oysters, more or less successfully, in order to stimulate them to produce jewelry. Thomas de Padova

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