Pearls are hard, smooth, and lustrous objects sometimes of gemstone quality which is produced within a living mollusk. Pearls can be produced by almost all mollusk species, however not all mollusks produce gemstone quality pearls.
Natural pearls that occur without human intervention are very rare which perhaps accounts for their high value. Traditionally, pearl divers would need to harvest hundreds of mollusks in the hope of finding a single pearl.
A pearl is created as a reaction to a foreign body or irritant such as a parasite or dislodged piece of the mantle (internal soft tissue). When an irritant becomes trapped, it stimulates the mollusk to begin a natural process that involves producing layers of calcium carbonate (nacre) around the object in the form of secretions. This process continues throughout the mollusc's life adding layer upon layer The thin layers of nacre and their light refracting quality is what give the pearl its highly prized lustrous appearance.
Nowadays, the vast majority of pearls are cultured or farmed. The process was first developed by William Savile-Kent in Australia and was then taken to Japan where it was patented by Tokichi Nishikawa in 1916. The process of pearl farming involves the artificial introduction of an irritant into a live mollusk. If the implant is successful the mollusk will then begin to react to the irritant much as it would in the wild and therefore produce a pearl.
Not all implants are successful and some mollusks do not survive. However, the process of farming or culturing pearls means that mollusks may produce several pearls at once. Cultured pearls make up the majority of the world's commercial pearls. The process of pearl farming is slow, with a pearl (depending on the species of mollusk) taking between 2 and 7 years to grow.