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.