Mother of Pearl (MOP), or Nacre is an organic-inorganic composite material produced by some mollusks as an inner shell layer. It is also what makes up pearls. It is very strong, resilient, and iridescent. Nacre appears iridescent because the thickness of the aragonite platelets is close to the wavelength of visible light. This results in constructive and destructive interference of different wavelengths of light, resulting in different colours of light being reflected at different viewing angles.