mineral

Fire Agate

Chalcedony (Silicon Dioxide, SiO2) with Limonite/Goethite

Hardness: 6.5-7 (Mohs scale); Color: Brown with iridescent flashes of red, gold, green; Luster: Vitreous to waxy; Crystal structure: Trigonal/Microcrystalline; Cleavage: None; Specific Gravity: 2.58-2.64.

Hardness: 6
Color: Brown with iridescent flashes of red, gold, green
Luster: Vitreous to waxy

Identified April 6, 2026More mineral →

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Physical properties

Formation & geological history

Formed during periods of Tertiary volcanic activity when hot water saturated with silica and iron oxide crowded into cracks and bubbles in volcanic rocks and cooled, creating thin layers. Typically found in volcanic host rock.

Uses & applications

Predominantly used as a gemstone in jewelry (rings, pendants) and as a high-value collector's specimen due to its unique optical play of light.

Geological facts

The iridescent 'fire' is caused by the Schiller effect, where light is diffracted by alternating layers of silica and iron oxide (limonite) only a few molecules thick.

Field identification & locations

Identify in the field by its botryoidal (grape-like) growth habit and iridescent internal flashes. It is primarily found in the Southwestern USA (Arizona, California) and Northern Mexico.

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