a feldspar mineral in the plagioclase (q.v.) series that is often valued as a gemstone and as ornamental material for its red, blue, or green iridescence. The mineral is usually gray or brown to black and need not be iridescent; when used as a gem it is usually cut en cabochon (with a rounded convex surface). Labradorite is one of the more common plagioclase varieties and occurs in many gabbros, dolerites, norites, and basalts. anorthosite, a rock composed mainly of iridescent labradorite crystals up to about 2 m (6 to 7 feet) long, occurs in many of the world's mountain regions. Labradorite is named for its occurrence near Nain, on the coast of Labrador, Can.