iridescence, interference of light either at the surface or in the interior of a material that produces a series of colours as the angle of incidence changes. Best known are the colours seen in precious opal resulting from the interference of light by submicroscopic layers of nearly spherical particles 1,500–3,000 angstroms in diameter that are arranged in a regular pattern. Common opal lacks this layering, and scattered light merely gives a milky opalescence. Internal iridescence is due to closely spaced fractures or lamellae such as planes of differing composition caused by exsolution. Most familiar are the colours of labradorite and peristerite resulting from lamellae about 1,000 angstroms thick. Surface iridescence occurs on some ore minerals (e.g., hematite and sphalerite).