mineral

Fire Agate (Rough)

Silicon Dioxide (SiO2) - Variety of Chalcedony

AI-generated identification · may be incorrect

Hardness: 6.5-7 (Mohs); Color: Brownish-red exterior often hiding iridescent flashes; Luster: Waxy to vitreous; Crystal structure: Trigonal/Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.58-2.64.

Hardness: 6
Color: Brownish-red exterior often hiding iridescent flashes
Luster: Waxy to vitreous

Identified February 21, 2026More mineral →

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Physical properties

Formation & geological history

Formed through volcanic activity during the Tertiary Period (roughly 24-36 million years ago). Created when hot water saturated with silica and iron oxide seeped into cavities in volcanic rock, cooling to form layers of chalcedony with iron oxide (limonite) thin films that create diffraction.

Uses & applications

Primarily used in jewelry making (cabochons), as metaphysical stones, and as unique mineral specimens for collectors. Due to its hardness, it is also suitable for carving.

Geological facts

The iridescent 'fire' effect is caused by the Schiller effect, resulting from alternating layers of silica and iron oxide that interfere with light. It is one of the more difficult gemstones to cut because the lapidary must follow the contours of the 'fire' layers without grinding through them.

Field identification & locations

Identify in the field by its distinctive botryoidal (grape-like) bubbles and brownish, waxy exterior often found in desert regions. It is primarily found in the Southwestern United States (Arizona, California) and Northern Mexico.