An iridescent fracture is a fracture or break inside a gemstone whose surface displays interference colours when viewed from certain angles. The colours, typically vivid rainbow hues, arise from thin-film interference where the fracture surfaces have separated by a small distance comparable to the wavelengths of visible light, trapping a thin film of fluid, gas or air between them.

Origin

Fractures in gem materials are irregular breaks rather than crystallographically controlled cleavages, but their internal surfaces, if they have separated by the right distance and remain reasonably smooth, can produce the same kind of thin-film interference that creates the colours of an oil slick on water. Light reflecting from the top and bottom of the gap selects which wavelength survives constructive addition at each viewing angle, producing single saturated colours that change as the stone tilts.

Identification and reporting

Iridescent fractures are typical clarity characteristics in materials that fracture readily, including quartz varieties, beryl, tourmaline and topaz. Laboratory reports plot them as feathers or fractures with a notation about the iridescence. The visual effect can be considered enhancing (some quartz with iridescent fractures is sold as rainbow quartz) or detrimental, depending on the material, the size of the fracture and the position relative to the table of the cut stone.

Practical considerations

A stone with a substantial iridescent fracture should be set with care because the fracture represents a structural weakness; impact at the wrong angle can cause the fracture to extend. The iridescence does not by itself indicate any specific risk beyond the underlying fracture's mechanical consequences, but it does serve as a visible flag that a fracture exists.