What is a pearl?
Pearls are the result of a defensive mechanism used by certain molluscs, such as oysters and mussels, when an irritant finds its way into their shells. This irritant could be a tiny burrowing crab or a piece of dead coral, almost anything for that matter.

The mollusc will then defend itself from this irritant by covering the foreign object with nacre.  Nacre is a naturally iridescent substance and contains the same range of colours which also lines the molluscs’ shells. This mother of pearl inside some freshwater molluscs’ shells has been used for centuries to make jewellery, buttons and furniture inlay amongst other things. 

When something finds it’s way into a mollusc and is then covered by layers of nacre during several years the resulting lustrous object that comes out is called a pearl.