Play-of-Color

Play-of-color is the defining characteristic of precious opal and is entirely distinct from iridescence. The silica spheres in precious opal are uniform in size (150–350 nm) and packed in a regular three-dimensional grid. When the sphere diameter is appropriate, visible light wavelengths are diffracted, producing pure spectral colors that shift with movement. Sphere size determines color range: smaller spheres produce violet and blue; larger ones produce red. Red play-of-color is rarest and most valued. Black opal from Lightning Ridge, Australia, shows the most vivid play-of-color against its dark body color.