1.8" Polished Fire Agate - Calvillo Mines, Mexico

Fire agate is a variety of chalcedony that often displays a metallic, shimmering iridescence known as adularescence or the Schiller Effect. It likely forms by means of fluid evaporation within a cavity that is already experiencing chalcedony deposition. In these cases, a thin iridescent film floating on the stagnant, silica-rich fluids forms as ferrous iron oxidizes when coming in contact with air in the cavity, producing ferric iron (Fe202). Once the cavity experiences a dry period, the film collapses onto the pre-existing chalcedony, depositing the iridescent layer that creates the fire effect. A following chalcedony band encloses the iridescent film inside the agate. This process can repeat over multiple periods of hydration and dehydration within the pocket.