sedimentary
Chert (specifically Flint or Jasper variant)
Microcrystalline quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Dark brown to black with a light grey weathered cortex, Luster: Waxy to dull, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Dark brown to black with a light grey weathered cortex, Luster: Waxy to dull, Crystal Structure: Trigonal (microcrystalline), 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 chemical precipitation in marine environments. The specimen shows a dark interior with a white/grey chalky outer weathering layer known as a cortex.
Uses & applications
Historically used for stone tools (arrowheads and knives) due to sharp conchoidal edges; also used in flintlock firearms and currently as decorative landscape stones or lapidary tumbling.
Geological facts
Chert is so durable that it often survives as rounded pebbles in riverbeds or beaches long after the surrounding limestone or chalk has eroded away. It was one of the first materials used by early humans for tool making.
Field identification & locations
Identify by its characteristic 'shell-like' conchoidal fracture patterns when broken and its ability to scratch glass. Found globally in sedimentary basins, often embedded in limestone or chalk beds.
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