Labradorite is a Plagioclase Feldspar found in magmatic igneous and metamorphic rock.  Feldspars are extremely abundant in the Earth's crust, the most common varieties falling into two groups - the Alkali Feldspars and the Plagioclase Feldspars. 

Plagioclase Feldspars (also called Soda-Lime Feldspars) form in a series from "pure sodium to pure calcium end members as follows: Albite - Oligoclase - Andesine - Labradorite - Bytownite - Anorthite."  The first to crystalize from high temperature molten rock are the calcium rich Plagioclase Feldspars.  As a molten mass cools, the sodium becomes concentrated, and the final Plagioclase Feldspar (Albite) then forms.  Labradorite lies somewhat in the middle of this series. 

 Discovered in 1770 on St. Paul Island, Labrador, Canada, Labradorite was named for this site - a locale famous for producing the variety of stone containing the iridescent schiller effect - in this instance called "labradorescence". 

Typically grey to black in colour, Labradorite often reveals its iridescent display in either violet, blue, green, gold, orange, yellow or red colour, or a combination of these.  Like its sister the Moonstone, the iridescence, or "labradoresence", of Labradorite is caused by thin parallel layers (called lamellae) within the stone.  These lamellae result from the separation and layering that takes place when chemistries change as high temperatures cool.  The layers formed then reflect the light that enters the stone but before doing so will refract it back and forth through the various layers, scattering it, until it emerges from the stone at a different wavelength than what it originally entered at; this is called diffraction.  The new wavelength will be the wavelength of a different colour.  If layers are too thick or too thin, or if not viewed from the precise angle that will diffract the light, then no labradorescence will be visible. 

This gemstone is also found in Australia, Finland, India, Madagascar, Mexico, Newfoundland, Norway, Russia and the United States