Raw Fire Agate from Arizona

This is a piece of raw fire agate I found while out rockhounding in the Arizona desert.

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Arizona fire agate forms through a process initiated by volcanic activity that occurred approximately 24 to 36 million years ago during the Tertiary Period. This activity created cavities, such as gas bubbles in lava flows, within volcanic rocks like basalt and andesite. Hot, mineral-rich water, saturated with silica and iron oxide, then seeped into these cavities. As the water cooled, it deposited layers of chalcedony, a microcrystalline variety of quartz, and crystallized iron oxide, such as goethite or limonite, within the voids. The formation can occur through the slow buildup of thin layers from watery fluids or more rapidly from viscous silica gels. The resulting structure consists of super-thin, alternating layers of silica and iron oxide, typically around 0.5 microns thick, which are responsible for the stone's iridescent "fire" effect. This iridescence is caused by the Schiller effect, where light diffracts and interferes within these microscopic layers, creating flashes of red, orange, yellow, and sometimes green or blue. The process often results in botryoidal (grape-like) or bubble-shaped formations within the host rock.