Google for a "reflective diffraction grating". If you shape the surface a certain way - grooves with a size near the visible wavelength, AFAI understand - you c

Google for a "reflective diffraction grating". If you shape the surface a certain way - grooves with a size near the visible wavelength, AFAI understand - you can get the reflected light to interfere with itself. The imperfections which vary the groove size then result in this rainbow effect.