sedimentary
Chert (specifically Flint or Dark Chert)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: Black to dark grey, Luster: Waxy to dull, Crystal structure: Microcrystalline/cryptocrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Black to dark grey, Luster: Waxy to dull, Crystal structure: Microcrystalline/cryptocrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of siliceous remains of organisms or chemical precipitation of silica in sedimentary environments. Often found as nodules in limestone or chalk beds.
Uses & applications
Historically used for stone tools (knapping) and fire-starting. Currently used as a construction aggregate, in road surfacing, and as a decorative stone in landscaping.
Geological facts
Because of its conchoidal fracture, it breaks into pieces with very sharp edges, leading prehistoric humans to use it for arrowheads and scrapers. It was also vital for fire-starting in flintlock firearms.
Field identification & locations
Identify by its extreme hardness (scratches glass), waxy texture when freshly broken, and the presence of curved, shell-like (conchoidal) fracture patterns. Found globally in sedimentary basins.
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