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Scanning electron micrograph of amorphous silica spheres closest packed in an
Australian fire opal showing red interference colors. Sphere size is ~2500
Ã…ngstoms. Photo courtesy of Hans-Ude Nissen. Cover of Reviews in Mineralogy
vol. 29 "Silica"
This regular array of balls of opal act like a diffraction grating that will refract various wavelengths (approximately the same wavelength as the diameter of the spheres). Different sizes of spheres will diffract different wavelengths of light thus giving rise to the various flash of different colors.
Precious opal cabochons (largest is 15mm), Australia. These are triplets, formed
with a dark background layer, a middle layer of precious opal and a surface
layer of quartz to help protect the softer opal from wear and tear.