sedimentary
Chert Pebble
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Dark gray to olive green with brown weathering, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6
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Physical properties
Hardness: 7 (Mohs scale), Color: Dark gray to olive green with brown weathering, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6
Formation & geological history
Formed by the precipitation of silica from seawater or the accumulation of silica-rich organic remains (like radiolarians) in deep-sea environments. This specimen has been rounded by fluvial or marine abrasion.
Uses & applications
Historically used for making stone tools (flintknapping). Professionally used in construction aggregate or as decorative gravel. Small river-worn samples are often collected by hobbyists.
Geological facts
Chert is essentially the same mineral as flint, jasper, and agate. It is highly resistant to chemical weathering, which is why it often remains as rounded pebbles in gravel beds long after other rocks have eroded.
Field identification & locations
Identify by its high hardness (it will scratch glass) and its smooth, waxy texture. Look for conchoidal (shell-like) fractures if the rock is chipped. Commonly found in riverbeds, glacial tills, and coastal beaches.
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Cone-in-cone structure (Secondary Sedimentary Structure)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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