Feature Gems & Gemology, Winter 2006, Volume 42, No. 4

The Cause of Iridescence in Rainbow Andradite from Nara, Japan

“Rainbow” andradite from Nara, Japan, occurs as relatively small orangy brown crystals that show attractive iridescence in almost the entire range of the spectrum. The material is nearly pure andradite, as determined by its chemical composition and Vis-NIR and specular reflectance FTIR spectra. Microscopy revealed that two different types of lamellar structures appear to be responsible for the iridescent colors. These lamellar structures cause predominantly thin-film interference and most likely diffraction of light. The terms interference and diffraction are explained and correlated to the structure and iridescence observed in these garnets.