mineral

Polished agate geode halves

Chalcedony (microcrystalline quartz, SiO₂), commonly with quartz and iron-oxide inclusions

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; gray, cream, brown, and reddish bands with a waxy to glassy luster and fine concentric or botryoidal structure. The cut surfaces expose translucent chalcedony and a small drusy quartz-lined cavity.

Identified September 20, 2026More mineral →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater fills cavities in volcanic rocks, depositing chalcedony and quartz in layers; ages range from Paleozoic to Cenozoic depending on the deposit. Common sources include Brazil, Uruguay, Mexico, Madagascar, and the United States.

Uses & applications

Used as cabochons, bookends, display specimens, and ornamental slices; agate is also used for beads, carvings, and industrial abrasives. These small polished halves have modest decorative value.

Geological facts

Agate’s banding records repeated changes in fluid chemistry during cavity filling. Iron oxides produce the brown, red, and orange colors.

Field identification & locations

Identify by concentric banding, waxy chalcedony, conchoidal fracture, and resistance to scratching by steel; it should not fizz in dilute acid. Common field material occurs as rounded nodules in basalt and volcanic ash.