sedimentary
Chert (often called Flint or River Stone)
Microcrystalline alpha-quartz (SiO2)
Hardness: 7 (Mohs scale), Color: dull brownish-grey, Luster: waxy to dull, Crystal structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: dull brownish-grey, Luster: waxy to dull, Crystal structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation in marine or groundwater environments. Often found as nodules within limestone or chalk.
Uses & applications
Used historically for stone tools and arrowheads; today used in construction aggregate, road base, and as decorative landscape stone.
Geological facts
Chert breaks with a conchoidal fracture, creating sharp edges that made it the primary material for prehistoric survival tools globally. It is extremely resistant to chemical weathering.
Field identification & locations
Identify by its extreme hardness (scratches steel/glass), smooth waxy feel, and curved 'shell-like' edges when broken. It is extremely common in riverbeds and areas with exposed limestone.
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