The mineral structure, chemical compositions and spectral characteristics of rainbow garnet samples from Tenkawa, Yoshino area, Nara Prefecture, Japan, were studied by Scanning Electron Microscopy with Energy Dispersive Spectrometer, Electron Microprobe, Fourier Transform Infrared Spectrometer and Laser Raman Spectrometer. In this paper, the authors analyzed the factors responsible for the rainbow effect and interpreted rainbow garnet's formation environment. The backscattering image shows stripes of different shades, the semi-quantitative analysis of the energy spectrum proves that the elemental composition of light gray stripe is the same as that of pure andradite, and the dark gray stripe's elemental composition likes that of Al-containing andradite. The two components form a lamellae structure, which causes interference and diffraction of light to produce a rainbow effect. Electron microprobe analysis makes sure that the main component of Japanese rainbow garnet is close to that of pure andradite. On the surface of the iridescent mineral (rainbow face), the vertical lamellae structure forms a diffraction grating, which causes the incident light to undergo grating diffraction. The lamellae of parallel rainbow growth cause interference of incident light. The light waves generated by the above two actions combine to form the iridescent color. The peak position and the Fe-O peak position are affected by a small amount of Al-O structure in the reflection infrared spectrum, which proves that the infrared spectrum characteristics of the Japanese rainbow garnet correspond to those of the andradite structure containing a small amount of Al. The laser Raman spectroscopy test reveals that the peak position of the high Al moves 2 cm^-1 in the high frequency direction compared with that of the less Al content (or none), and the overall spectral characteristics are consistent with the structural features of the andradite. Japanese rainbow garnet is intercalated with andradite and Al-bearing andradite. It is inferred that it was formed in an environment rich in Fe and depleted in Al.