In opal, play-of-color is produced primarily by what mechanism?

• A. Impurities causing color
• B. Diffraction by a regular array of silica spheres
• C. Fluorescence under UV light
• D. Structural defects causing color

Answer: (B)

Play-of-color in opal comes from light diffraction in the stone’s internal microstructure. The opal contains a regular, close-packed array of silica spheres that form a nanostructured lattice. When white light hits this arrangement, it is scattered and interferes constructively at specific wavelengths. The orderly spacing of the spheres acts like a photonic crystal, so certain colors are reinforced while others are diminished, producing vivid spectral colors that shift with the viewing angle. The size and packing of the spheres set which colors appear, and the effect is a true diffraction/interference phenomenon, not caused by impurities, UV fluorescence, or simple defects.