sedimentary
Chert (Nodule)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: Brown to dark grey, Luster: Waxy to dull, Crystal structure: Cryptocrystalline (microscopic quartz), Cleavage: None (conchoidal fracture), Specific gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Brown to dark grey, Luster: Waxy to dull, Crystal structure: Cryptocrystalline (microscopic quartz), Cleavage: None (conchoidal fracture), Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation in marine environments; often found as nodules within limestone or chalk beds.
Uses & applications
Historically used for stone tools and weapons due to sharp fractures; currently used for road surfacing, as grinding media, or as decorative landscaping stones.
Geological facts
Chert has a conchoidal fracture which allows it to break into very sharp edges, a property exploited by prehistoric humans to make arrowheads and scrapers.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by steel) and smooth, shell-like fracture pattern. Common in sedimentary basins worldwide.
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