is composed of hexagonal platelets of aragonite ( a type of calcium carbonate) (in a crystalline form) and conchiolin (an organic protein substance which provides bonding). The specific lustre, iridescence, and colouring of nacre — and, therefore, of any pearl which it forms — depends on the number and thickness of the various layers, as well as on whether or not the layers overlap one another. A freshwater pearl is made up of many layers of nacre and no bead.
The layers of nacre can be clearly seen in this freshwater
solid nacre pearl which has split open
A pearl which is ‘wild’ ie one which has grown without any human activity or intervention is called a natural pearl. Natural pearls are very rare these days and so command high prices. There are still several wild colonies of freshwater mussels Margaritifera margaritifera in the UK and they are protected by law under the Wildlife and Countryside Act 1981. Selling a UK natural pearl needs a licence. These molluscs can live for over 100 years. The earliest reference in Britain to freshwater mussels is by Julius Caesar’s biographer, Suetonius, who stated that Caesar’s admiration of pearls was a reason for the first Roman invasion in 55BC.