opalescence

thumb|Rough sample of common opal thumb|Rough sample of precious opal, showing iridescence

Opalescence or play of color is an optical phenomenon associated with the mineraloid gemstone opal, a hydrated silicon dioxide. This effect appears as a milky, translucent glow that changes with the angle of light, often creating a soft, pearly sheen that can display various colors or hues. Opalescence can be seen in materials like certain minerals, glass, and even fluids.