Diffraction gratings are composed of a series of ridges spaced very closely and regularly, and this physical shape is transferred to the chocolate through the m

Diffraction gratings are composed of a series of ridges spaced very closely and regularly, and this physical shape is transferred to the chocolate through the molding process. Light bouncing off of the different ridges creates an interference pattern which varies for different wavelengths of light, creating a rainbow effect. https://en.m.wikipedia.org/wiki/Diffraction_grating https://en.m.wikipedia.org/wiki/Diffraction_grating