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.

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