How do pearls form?

Pearls develop inside molluscs – including oysters, mussels and clams – whenever a foreign particle enters its shell and irritates the soft inner tissues. If the animal can’t expel the irritant, it will engage a unique defence mechanism. To protect itself from the particle, the mollusc produces a substance called nacre, or mother-of-pearl, which also lines the inner surface of the creature’s shell. Layer upon layer of the hard crystalline nacre is then used to smother the invading object so it cannot harm or contaminate the mollusc. A pearl’s iridescent appearance is due to the many layers of nacre that consist of many microscopic crystals. The thickness of one layer of calcium carbonate plates is similar to the wavelength of visible light. Some of the light passing through the top layer of nacre will be reflected, but some will continue to travel through to the bottom layer where further light is reflected. Multiple reflections interfere with each other at different wavelengths, causing colours to be reflected and scattered in all directions, creating an iridescent finish.