Nacre is a pearl’s outer layer, made up of aragonite deposits. Aragonite is a form of calcium carbonate that is deposited in layers over the pearl’s surface. Thicker nacre deposits tend to create stronger, more resilient pearls, as well as the deep, warm luster seen most commonly in South Sea pearls. Thinner nacre layers often result in shinier, more mirror-like surfaces, like those of Akoya pearls. Very thin nacre is considered undesirable in gem quality pearls.
Nacre is iridescent in nature, as the thickness of the aragonite layers is similar to that of the wavelength of visible light. This iridescence lends color overtones to pearls, and can be highly individual pearl to pearl.