Labradorite is an unusual mineral. It can display a beautiful iridescent play of colors, caused by internal fractures in the mineral that reflect light back and forth, dispersing it into different colors.
Labradorite is a feldspar mineral of the plagioclase series that is most often found in mafic igneous rocks such as basalt, gabbro, and norite. It is also found in anorthosite, an igneous rock in which labradorite can be the most abundant mineral.
Some specimens of labradorite exhibit a schiller effect, which is a strong play of iridescent blue, green, red, orange, and yellow colors as shown in the photographs. Labradorite is so well known for these spectacular displays of color that the phenomenon is known as "labradorescence." Specimens with the highest quality labradorescence are often selected for use as gemstones.
Labradorescence is not a display of colors reflected from the surface of a specimen. Instead, light enters the stone, strikes a twinning surface within the stone, and reflects from it. The color seen by the observer is the color of light reflected from that twinning surface. Different twinning surfaces within the stone reflect different colors of light. Light-reflecting from different twinning surfaces in various parts of the stone can give the stone a multi-colored appearance.
Labradorite has become a popular gemstone because of the unique iridescent play-of-color that many specimens exhibit. The quality, hue, and brilliance of the labradorescence vary from one specimen to another and within a single specimen. Stones with exceptional color are often given the name "spectrolite."