sedimentary

Chert (Chalcedony / Agate Pebble)

Silicon Dioxide (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale. Color: Brownish, greyish-tan, with waxy to dull luster. Conchoidal fracture, microcrystalline structure, no distinct cleavage. Specific gravity: ~2.65.

Hardness: 7 on Mohs scale
Color: Brownish, greyish-tan, with waxy to dull luster

Identified July 26, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale. Color: Brownish, greyish-tan, with waxy to dull luster. Conchoidal fracture, microcrystalline structure, no distinct cleavage. Specific gravity: ~2.65.

Formation & geological history

Formed through chemical precipitation of silica or accumulation of siliceous organism remains (like radiolarians or diatoms) in marine or freshwater sedimentary environments, often smoothed into a pebble by stream or beach erosion.

Uses & applications

Historically used for flaking stone tools (arrowheads, fire-starting sparkers). Modern uses include tumbled stones for hobbyists, landscaping, and aggregate.

Geological facts

Chert is extremely durable due to its fine-grained quartz structure. Darker varieties with smooth textures were historically used as touchstones to test the purity of gold and silver.

Field identification & locations

Look for a smooth, waxy surface, sharp shell-like (conchoidal) fractures where chipped, and high hardness (scratches glass). Widely found in riverbeds, gravel pits, and coastal beaches.