Natural Materials: Pearls

Pearls are created by many molluscs although few of them produce desirable pearls such as those produced by pearl oysters. Pearls are made up of many layers of nacre which is produced by molluscs as a defence response to irritants such as parasites. Naturally produced pearls are rare and most pearls produced today are cultured. Cultured pearls are created through the planting of a nucleus, such as a bead made from shell and/or mantle tissue into the host mollusc. The pearl is then gradually formed as the mollusc builds layers of nacre around the nucleus or tissue when it's natural defence response is triggered. This may be natural or induced artificially in order to create the pearl more quickly.

Freshwater pearls are created by molluscs which live in lakes, rivers, ponds and other bodies of freshwater. They can reside in hot or cool climates and are often cultured using tissue only.

X-rays reveal how a pearl has been created, for example, a natural pearl will show concentric layers right to it's core, a cultured pearl created using a nucleus or bead will show a solid centre surrounded by the layers of nacre created by the mollusc. If mantle tissue without a bead is used, it is more difficult to distinguish if the pearl is natural.

Pearls are categorized according to size and shape and cultured freshwater pearls can be dyed. The lustre of a pearl is generally related to the thinness and number of layers.