I

ikecho-gai

 Japanese name for the Biwa pearly mussel, Hyriosis schlegeli

interference

 creates the iridescent play of color on the surface of a pearl; refraction, diffraction, and thin-film interference occur as light penetrates extremely thin nacre layer

iridescence

 play of color over the surface of a pearl as interference divides white light into its component colors, caused by the overlapping of successive layers

irritant

a minute snail, worm, fish, crab or any other small particle that begins the formation of the pearl