mineral

Tumbled banded agate

Microcrystalline quartz (chalcedony), SiO₂

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; rounded polished pebble with gray, brown, cream, and bluish-gray curved bands, vitreous-to-waxy luster, cryptocrystalline structure, and no visible cleavage. The banding likely reflects alternating chalcedony and quartz layers.

Identified August 26, 2026More mineral →

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Physical properties

Formation & geological history

Agate forms when silica-rich groundwater fills vesicles in volcanic rocks, depositing concentric layers over time; individual specimens commonly formed during the Cenozoic or older volcanic episodes. This piece has been cut or tumbled and polished.

Uses & applications

Used for cabochons, beads, carvings, ornaments, and inexpensive jewelry; also collected as a decorative stone. Typical value is about $2–$20, or $20–$75 for especially attractive patterning and polish.

Geological facts

Agate is a variety of chalcedony, itself a microcrystalline form of quartz. Its bands may be straight, curved, mossy, or fortification-like depending on cavity geometry and fluid chemistry.

Field identification & locations

Identify it by its waxy polish, translucent edges, fine curved banding, and quartz-like hardness; it should scratch glass but not be scratched by steel easily. Common sources include Brazil, Uruguay, Mexico, India, Madagascar, and the western United States.