mineral

Polished brown agate or chert pebble

Microcrystalline quartz (SiO₂), likely chalcedony/agate or jasper-rich chert

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; tan-brown with darker reddish-brown zones, waxy to dull luster, and no visible cleavage. Fine-grained, cryptocrystalline quartz; the smooth surface indicates tumbling or natural abrasion, while the cavity may be a weathered or vuggy area.

Identified August 26, 2026More mineral →

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Physical properties

Formation & geological history

Silica-rich groundwater formed chalcedony or agate by filling cavities and fractures in volcanic rock; chert formed by replacement or accumulation of silica in sediment. Such silica deposits range from Paleozoic to Cenozoic in age, depending on locality.

Uses & applications

Used as a decorative stone, tumbled stone, cabochon, and lapidary material; harder varieties can be polished to a high sheen.

Geological facts

Agate commonly shows concentric banding, though this specimen’s subdued pattern may be concealed by its opaque brown coloration. A steel file should not scratch it easily, but glass is typically scratched by quartz.

Field identification & locations

Common in volcanic terrains, stream gravels, and silica-rich sedimentary formations worldwide, especially Brazil, Uruguay, Mexico, India, and the western United States. Confirm with a hardness test, translucent edges, and close inspection for banding.