This drawing shows you a more detailed look at how nacre is formed. A mother of pearl shell bead nucleus is inserted into the host oyster, along with a square of donor mantle tissue from another oyster who has previously borne pearls of high or exceptional value, and the oyster is left to begin secreting around the nucleus, eventually forming a pearl.
The crystalline nacre layers are made up of microscopic aragonite platelets composed of calcium carbonate, and bound together with an organic “glue” of sorts called conchiolin. The aragonite platelets are fairly transparent to slightly tinged in color, able to both reflect and refract light rays striking their surfaces, giving pearls their trademark pearl lust and subtle glow. The tighter and more compact this arrangement of crystal is, the brighter the luster the pearl displays and the greater the amount of orient or iridescence.