mineral

Fire Agate

Silicon Dioxide (SiO2) with Goethite or Limonite inclusions

Hardness: 6.5-7 (Mohs scale), Color: Brown to reddish body with vibrant iridescence (red, orange, green, gold), Luster: Vitreous to waxy, Crystal Structure: Trigonal (Cryptocrystalline), Cleavage: None (Conchoidal fracture)

Hardness: 6

Identified June 28, 2026More mineral →

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Physical properties

Formation & geological history

Formed during Cenozoic volcanic activity when silica-rich hydrothermal solutions filled cracks and cavities in volcanic rock. The iridescent effect results from thin layers of iron oxide (Goethite) deposited during the growth of chalcedony.

Uses & applications

Primary use is in high-end jewelry as cabochons, and as a prized specimen for gemstone collectors and lapidary artists.

Geological facts

The iridescent 'fire' is caused by the diffraction and interference of light as it passes through the micro-layers of iron oxide and silica. It is often found in botryoidal (grape-like) formations.

Field identification & locations

Found almost exclusively in the Southwestern USA (Arizona, California) and Northern Mexico. In the field, look for nodules with a bubbly surface and hints of red or gold flash when held to sunlight. Most high-quality material is excavated from Tertiary-age volcanic rhyolite or andesite beds.

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