sedimentary
Chert (River Siltstone/Flint)
Chert (SiO2)
Hardness: 7 (Mohs scale), Color: light grey to cream with tan banding, Luster: dull to waxy, Crystal structure: microcrystalline/cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.5-2.8.
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Physical properties
Hardness: 7 (Mohs scale), Color: light grey to cream with tan banding, Luster: dull to waxy, Crystal structure: microcrystalline/cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.5-2.8.
Formation & geological history
Formed by the accumulation of microscopic silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in marine or freshwater environments. This specimen shows significant water-worn smoothing, likely originating from a riverbed.
Uses & applications
Historically used for stone tools and arrowheads; currently used in construction aggregate, road surfacing, and as decorative river stones.
Geological facts
Because it fractures conchoidally (like glass), chert was one of the most important minerals for early humans to create sharp cutting tools. It is highly resistant to chemical weathering.
Field identification & locations
Identify by its high hardness (it will scratch glass and steel), its smooth water-worn surface, and its waxy luster when freshly broken. Commonly found in riverbeds, glacial till, and sedimentary outcrops worldwide.
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