How does play-of-color differ in opal from other gem colors?

• 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 by its internal, regularly arranged silica spheres. The tiny spheres are packed in a way that acts like a diffraction grating, so light is scattered and interferes to separate into spectral colors. As you view the stone from different angles, the constructive interference shifts, causing the shifting rainbow flashes you see.

This is different from most gem colors, which stem from pigments or impurities that absorb certain wavelengths, or from fluorescence under UV light, or from color centers created by defects. In opal, the color isn’t due to pigment or defects but to the physical structure that diffracts light.