Ammolite is a kind of organic gemstone that often shows red, green and blue iridescence after coating. It is mainly obtained from iridescent fossilized ammonite shells mined in Alberta, Canada. In this paper, the authors studied the samples by conventional gemological testes,Raman spectrum, UV-Vis spectrum,and scanning electron microscope. The gemological characteristics, chemical composition, surface micro-topography and color origin were analyzed. The results show that the split and coated layer can be observed under the microscope. The surface colors in different areas of the ammonite are closely related to the layer structure. With thicker and tighter aragonite layers, the color of ammonite shifts toward longer wavelength in the visible region. According to the results obtained, the color of ammonite is probably caused by interference effect of the aragonite layers.