sedimentary
Chert / River Cobble
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs scale), Color: tan, grey, and black banding, Luster: waxy to dull, Crystal structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific gravity: 2.6
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs scale), Color: tan, grey, and black banding, Luster: waxy to dull, Crystal structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific gravity: 2.6
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation in marine environments; subsequently rounded and smoothed by water transport in a river or coastal environment.
Uses & applications
Used historically for stone tools and today for road construction, landscaping, and as decorative river stones.
Geological facts
This specimen shows distinctive black veining or 'pressure solution' lines called stylolites, or possibly carbonaceous inclusions. Chert is a common find in riverbeds because it is harder than most surrounding rocks and survives long-distance transport.
Field identification & locations
Identify by its hardness (cannot be scratched by a steel knife), waxy luster on fresh surfaces, and smooth, water-worn exterior. Commonly found in gravel pits and along riverbanks.
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