sedimentary
Chert (Flint)
Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: tan, brown, or brownish-gray, Luster: waxy to dull, Structure: microcrystalline, Fracture: conchoidal (shell-like), Cleavage: none.
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, brown, or brownish-gray, Luster: waxy to dull, Structure: microcrystalline, Fracture: conchoidal (shell-like), Cleavage: none.
Formation & geological history
Formed via the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in sedimentary environments. Common in limestone deposits as nodules.
Uses & applications
Historically used for stone tools and arrowheads; currently used in construction aggregate, road surfacing, and as a component in ceramics. Significant in historical fire-starting.
Geological facts
Because it breaks with a conchoidal fracture creating sharp edges, it was the primary material for prehistoric tool-making. It produces sparks when struck against steel.
Field identification & locations
Identify by its characteristic smooth, curved (conchoidal) fracture surfaces and its ability to scratch glass. Widely found in riverbeds, fields, and gravel pits globally.
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Brecciated Chert (Microcrystalline Silica)
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