iridescence

A shiny rainbow effect caused by diffraction or thin-film interference.

Labradorite

Labradorite has become a popular gemstone because of the unique iridescent play of color that many specimens exhibit.  Labradorite is a feldspar mineral of the plagioclase series that is most often found in mafic igneous rocks such as basalt, gabbro and norite.  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 above. The Schiller effect is also seen in fire agate and mother of pearl. 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.

Source: http://geology.com/gemstones/labradorite/

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Fire Agate

A triangular brown stone with the front face cut off and polished revealing iridescent shimmering red, orange, yellow, and a little bit of green colors inside.

Photo of a polished fire agate found in Aguascalientes, Mexico by Rob Lavinsky/irocks.com retrieved from http://www.mindat.org/photo-180670.html

Fire agates contain iridescent (rainbow) colors that resemble fire.  The iridescence comes from the diffraction of light between the fire agate’s alternating layers of silica and iron oxide.  This is called the Schiller effect.  You can also see the Schiller effect in labradorite and mother-of-pearl (hmmm, could there be a post about labradorite coming soon?).  It’s difficult to photograph iridescence, so any photos you see are probably even more beautiful in person.  Fire agates are rare, but may be found in Arizona, northern Mexico, and other parts of the Southwest.  Like other agates, they rate 6.5-7 on the Mohs scale and are gorgeous when polished.