Iridescence is an optical phenomenon whereby the colours on a surface appear to change depending on the angle at which they're viewed or the angle at which they're illuminated. Iridescence is caused by multiple reflections in multi-layered, semi-transparent surfaces. Some examples of things that are relatively well known for being iridescent include bubbles, the wings of a butterfly and certain rocks and minerals. Gemstones including Mother of Pearl, rainbow moonstone, labradorite and opal are well known for being iridescent.
These butterflies are on display in the Harvard Museum of Natural History, Cambridge, Massachusetts, USA. Photo by Stone Mania ©