Interference is the wave phenomenon that occurs when two or more light waves meet and either reinforce or cancel one another. Where the crests of two waves coincide, the brightness adds (constructive interference); where a crest meets a trough, the waves subtract and the amplitude falls towards zero (destructive interference). Because visible light is composed of overlapping wavelengths, the geometry of the path difference selects which colours survive and which disappear, and the eye perceives the survivors as a pure spectral hue.

Origin in gemmology

In gem materials interference arises whenever light is split into two paths whose recombined difference is comparable to the wavelength of visible light, between roughly 400 and 700 nanometres. The two classical mechanisms are thin-film interference and diffraction by a periodic structure, and although the latter is strictly diffraction, the colours it produces are the result of the same constructive and destructive addition of waves.

Thin-film interference

A thin film of material whose thickness lies in the visible-light range produces colour because some light reflects from its top surface and some from its bottom surface. The two reflected beams travel slightly different distances and interfere on recombination. Iridescent cleavage planes, the rainbow stains on tarnished hematite, and the colours of certain chalcopyrite intergrowths all arise this way. The hue shifts with viewing angle because the path difference depends on geometry; this angular dependence is the visual signature of an interference effect and distinguishes it from absorption colour, which does not move on tilt.

Iridescence in feldspar

Labradorescence in labradorite, the metallic blues, golds and greens that flash from cleavage surfaces, is an interference effect produced by lamellar intergrowths of two compositionally distinct feldspars. The lamellae are spaced at hundreds of nanometres, and incident light reflecting from successive boundaries arrives in phase only at specific angles and wavelengths. Spectrolite from Finland is the same phenomenon expressed across a fuller spectral range. Adularescence in moonstone has a related but milder origin, with scattering at finer lamellae producing the soft floating sheen rather than discrete spectral colours.

Distinguishing interference from dispersion

Trade literature sometimes confuses interference with dispersion, the splitting of white light into spectral colours by a prism or by the facets of a brilliant-cut diamond. Dispersion separates wavelengths by refractive-index difference and produces a continuous fan of colour; interference selects discrete wavelengths and produces a single saturated hue at a time, changing as the stone tilts. The angular dependence is the simplest field test.

Practical relevance

For the working gemmologist, identifying interference as the colour mechanism rules out coloured-body absorption and points instead to a structural feature, a cleavage, a lamellar intergrowth or a thin film. That distinction is diagnostic for opal varieties, feldspars, fire agate and certain chalcopyrite specimens, and it is also why such stones must be cut and oriented to capture the geometry that produces their colour.