Labradorite is a variety of plagioclase feldspar found in igneous rocks and is characterised by brilliant colours although the surface of labradorite is usually grey or almost black.
Like Moonstone, labradorite displays a shiller of iridescent colour called labradorescence which is the result of lamellar intergrowths inside the crystal. These intergrowths result from compatible chemistries at high temperatures becoming incompatible at lower temperatures and thus separating and layering.
The resulting colour effect is caused by a ray of light entering a layer and being refracted back and forth by deeper layers. This refracted ray is slowed by the extra travel through the layers and mixes with other rays to produce a light ray coming out that has a different wavelength to that when it went in. The wavelength could correspond to the wavelength of a particular colour, such as blue.