mineral

Fire Agate

Silicon Dioxide (SiO2) with layers of Limonite/Goethite

Hardness: 6.5-7.0 (Mohs); Color: Brown base with iridescent flashes of red, gold, and green; Luster: Waxy to vitreous; Structure: Microcrystalline (trigonal); Cleavage: None (conchoidal fracture); Specific Gravity: 2.58-2.64

Hardness: 6
Color: Brown base with iridescent flashes of red, gold, and green
Luster: Waxy to vitreous

Identified March 25, 2026More mineral →

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Physical properties

Formation & geological history

Formed through volcanic activity during the Tertiary period. Hot water saturated with silica and iron oxide seeped into cavities in volcanic rock, cooling to form thin layers of chalcedony and limonite that produce iridescent interference colors.

Uses & applications

Primarily used in high-end jewelry as cabochons, for ornamental carvings, and as a highly sought-after collector's specimen.

Geological facts

The 'fire' effect is caused by the Schiller effect (thin-film interference) created by alternating layers of silica and iron oxide. Unlike most gems, it is found only in specific areas of the Southwestern United States and Northern Mexico.

Field identification & locations

Identify in the field by its botryoidal (grape-like) growth habit and brown exterior that reveals flashes of color when wet or polished. Key locations include Arizona, California, and various states in Mexico (Aguascalientes, San Luis Potosí).

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