Labradorescence observed in Gem labradorite has been studied by theoretical calculations on the basis of experimental results. The theoretical calculations were made based on kinematlcal theory of diffraction (assuming infinite layer model) as well as on dynamical electromagnetic theory (assuming finite layer model). The results of calculations based on the two models agree well with experimental results. The calculated shift of peak maximum of intensity reflectance, 2.2 (nm/degree), is well consistent with the experimental value, 1.5-1.9 (nm/degree). The effects of non-parallel lamellar boundaries and irregularities of lamellar thicknesses have been also treated. As the boundary roughness of lamella increases, more diffuse reflection is produced resulting in whitish colour with rernaining wavelength maximum constant. Therefore, Iabradorescence is the interference phenomenon of reflected waves by lamellar multilayer which consists of two periodically stacked lamellar structure. It can be understood that peak maximum of intensity of iridescent spectra is followed by Bragg condition.
