gemstone

Fire Agate

Silicon Dioxide with Limonite (SiO2 + FeO(OH)·nH2O)

Hardness: 6.5-7.0 (Mohs). Color: Reddish-brown with iridescent flashes of orange, green, and gold. Luster: Vitreous to waxy. Crystal Structure: Hexagonal (cryptocrystalline). Specific Gravity: 2.6-2.64.

Hardness: 6
Luster: Vitreous to waxy

Identified July 10, 2026More gemstone →

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Physical properties

Formation & geological history

Formed through volcanic activity where silica-saturated water filled cavities in volcanic rock. The iridescence comes from thin layers of iron oxide (limonite) trapped between silica layers. Found in Tertiary volcanic host rocks (approx. 24-36 million years old).

Uses & applications

Primarily used in high-end jewelry as cabochons, metaphysical collecting, and ornamental carvings.

Geological facts

The iridescent effect is caused by the Schiller effect, which is light interference within the thin films of iron oxide. It is found in only a few locations in the Southwestern United States and Northern Mexico.

Field identification & locations

Identify by its unique 'bubbling' or botryoidal brown surface that shows colorful flashes when tilted. Common locations: Arizona, California, and Mexico.

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