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.