Closeup of artificial opal, formed on fragments of microscope slides by drying little plastic beads. Everywhere that the beads happened to assemble themselves into regular arrays as they dried, you get iridescent rainbow colors - the “fire” of opal. The rainbow iridescence comes from the interference of light waves as they bounce off the different layers of plastic beads - you only get this kind of iridescence if the beads are about as small as the waves of light themselves. My lab is using opals like these as chemical sensors - turns out that if there are chemicals stuck to the surface of the opal, it changes the rainbow pattern that reflects from the opal.
Photo taken with iPhone camera and Easy-Macro lens.