Color of cultured pearls of P. fucata (Akoya oyster) can be changed by light irradiation, which has been applied to pearl processing. The effects of light irradiation on fluorescence and optical reflectance of pearls have been investigated experimentally. Changes in the fluorescence and reflectance spectra have been observed. These changes depend on the wavelength of light and are considered to be due to degradation of conchiolin (a kind of scleroprotein) and discolouration of pigments contained in the nacre (the surface layer around the core of a pearl).