opalescence

Opalescence is an optical effect in which a substance displays a milky, translucent to opaque appearance with a soft, pearly glow that can shift to blues, pinks, or greens when viewed from different angles. The term is commonly applied to opal, but it also describes similar luminosity seen in glass, plastics, and mineral grains. In gemstones, opalescence can indicate internal structure and composition, and it is sometimes used interchangeably with the adjective opalescent.

In opal, the phenomenon arises from the way tiny silica spheres are packed within the stone. The spheres diffract light, creating a diffuse body color and, in many opals, flashes of spectral color known as play of color. The size and arrangement of the spheres determine the intensity and hue of the opalescence. Other materials may show opalescence when microstructural features scatter and refract light, producing a milky glow without dominant color play.

Opalescence is a characteristic descriptor in gemology, mineralogy, and materials science. It contrasts with iridescence, which implies vivid, angle-dependent colors produced mainly by interference, and with adularescence, a bluish glow in moonstone produced by layered refraction. Observers may notice changes in opalescence with thickness, surface texture, and lighting, including backlighting that enhances translucency.

Historically, the term derives from opal, a gemstone famous for its play of color; today it is used broadly to describe similar visual effects in natural and man-made materials. In practice, opalescence denotes a soft, milky glow rather than a sharp rainbow display, though the two can occur together in opalescent stones.