mineral

Fire Agate

SiO2 (Silicon Dioxide with Iron Oxide inclusions)

Hardness: 6.5-7 (Mohs); Color: Brown base with iridescent flashes of red, gold, green, and orange; Luster: Vitreous to waxy; Crystal structure: Microcrystalline (trigonal); Cleavage: None (conchoidal fracture); Specific gravity: 2.58-2.64.

Hardness: 6
Luster: Vitreous to waxy

Identified March 29, 2026More mineral →

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Physical properties

Formation & geological history

Formed during the Tertiary period (roughly 24-36 million years ago) due to volcanic activity. Hot water saturated with silica and iron oxide seeped into cavities in volcanic rocks, cooling to form thin layers of limonite and goethite within the agate structure.

Uses & applications

Primarily used in high-end jewelry and lapidary arts. It is carved into cabochons to best display its 'fire' or iridescence. It is also highly prized by mineral collectors.

Geological facts

The iridescent 'fire' is not caused by chemicals but by the diffraction of light through alternating layers of silica and iron oxide (the Schiller effect). It is considered one of the rarest types of chalcedony.

Field identification & locations

Identify in the field by its botryoidal (grape-like) habit and distinctive iridescence when wet or polished. It is found almost exclusively in the Southwestern United States (Arizona, California) and Northern Mexico. Collectors should look for bubbles or 'eyes' of color.

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