sedimentary
Chert (Iron-stained)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Brownish-yellow to dull tan (due to iron oxide staining), Luster: Waxy to dull, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Brownish-yellow to dull tan (due to iron oxide staining), Luster: Waxy to dull, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or by chemical precipitation from silica-rich fluids in sedimentary basins. Hardens via diagenesis over millions of years.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to sharp edges; currently used for road aggregate, concrete, and occasionally as a landscape stone.
Geological facts
Chert has a conchoidal fracture, allowing it to break like glass into razor-sharp edges. It is one of the most common materials used by prehistoric humans for tool-making.
Field identification & locations
Identify in the field by its waxy luster, smoothness to the touch compared to surrounding rocks, and its ability to scratch glass or steel. Found globally in limestone and dolomite formations.
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