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Pearls, often used as gemstones, are the by-products of certain molluscs' protective mechanisms.



pearl, jewellery, semi-precious stone, mussel, parasite, mother-of-pearl, pearl fishery, animal, molluscs, Mollusca, biology

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Pearl oyster

Open pearl oyster

  • pearl

Shell structure

  • outer layer
  • middle layer
  • mother of pearl (nacre) - Iridescent (rainbow-like), due to its microscopic structure. It produces the pearl.

Mother of pearl

  • calcium carbonate platelet - Small, translucent platelets. Iridescence is caused by multiple reflections from its two surfaces.
  • protein - It links the calcium carbonate platelets.

Pearl formation

  • The parasite enters the mollusc.
  • The mollusc produces nacre to surround the parasite and thereby defend the soft tissues against it.


  • South Sea pearls
  • Freshwater pearls
  • Akoya pearls
  • Tahitian black pearls



The inner surface of some types of molluscs – the saltwater pearl oyster and the freshwater pearl mussel – consists of three layers, the innermost of these being nacre or mother of pearl.

Mother of pearl consists of small, translucent calcium carbonate platelets which reflect light from both of their surfaces and thus produce iridescence. The platelets are linked by protein molecules.

Growing pearls is a protective mechanism in these molluscs. When a parasite enters the shell, it is surrounded by several layers of nacre in order to isolate it. It might take as long as a decade to form a pearl the size of a pea. A similar process takes place when debris enters the mollusc with water: the mollusc prevents damage by isolating it with nacre.

Cultured or farmed pearls are produced by inserting a ‘nucleus’, such as a small bead, inside the mollusc to stimulate pearl formation. Pearls are considered valuable gems and have been used for making jewellery since ancient times.

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