Abstract: The latest researches on the origin of iridescence in the gems are synthetically reviewed.The results indicate that the iridescence is mostly related to the inner periodic lamellar structures in the gems and the thickness of the lamellar structures restricted within a certain scope,which are different from the variety of the gems.The optical principles of thin-film interference and diffraction are discussed in detail for analysing two optical models including thin-film interference and grating diffraction.The iridescence produced by the thin-film interference needs the thickness of lamellar structure within the certain nanometer range and the lamellar structure formed by exsolution of two phases,which the chemical compositions,refractive index and thickness of the lamellar structure are trinity.The iridescence produced by the grating diffraction just needs the lamellar structure arrayed regularly and the narrow edges which either amplitude or phase or both of incident light could produce periodic spatial modulation.