GEMS MADE BY LIVING CREATURES

 Pearls

Meet the jewellery-making molluscs that turn irritating specks of dirt into stunning stones.

    Pearls are smooth shiny gems that are commonly seen on a long string decorating elegant necks. They have been used as a fashion accessory since at least the 5^th century BCE by the ancient Romans and Egyptians. But the process that creates these mesmerising gems isn't geological --- they are made by slow-moving marine and fresh water animals as a kind of symbol of their suffering. 

     Molluscs such as oysters, mussels and clams produce pearls as a byproduct of defence. They are made when these aquatic creatures attempt to rid their bodies of parasites, sharp pieces of shell matter or other debris that gets trapped inside their shells when they open them to feed. They secrete calcium carbonate, which is the core material that makes up a mollusc's shell, in a crystallised form when a irritant is present. It's called aragonite, and combines with protein fibres to coat smooth layers around the foreign object. The result is a creamy-coloured solid ball that serves as a barrier between the object and the soft internal tissue of the mollusc.

     Today, most pearls aren't harvested naturally. Instead, humans place a suitable irritant into an oyster to trigger this response and increase pearl production rates.

     Pearls are the only gems made by living creatures.