Iridescence

Iridescence

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Iridescence is a visual phenomenon where by 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, butterfly wings and certain minerals and gemstones including Mother of Pearl, rainbow moonstone, labradorite and opal.  

Further Reading:
Examples of iridescence on Wikipedia