sedimentary
Chert (Black Flint)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: Black or dark grey, Luster: Waxy to vitreous, Crystal Structure: Microcrystalline/Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Black or dark grey, Luster: Waxy to vitreous, Crystal Structure: Microcrystalline/Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation within sedimentary rock layers, often limestone or chalk. It can date back to the Precambrian era through the Cenozoic.
Uses & applications
Used historically for stone tools and starting fires; currently used for decorative gravel, jewelry (cabochons), and as a raw material in some ceramic glazes.
Geological facts
Black chert is often called 'Flint.' Because it breaks with a conchoidal (shell-like) fracture, it creates edges sharper than a steel scalpel, making it the primary material for prehistoric arrowheads and knives.
Field identification & locations
Identify in the field by its smooth, waxy surface and sharp, curved fracture edges. It will scratch glass and often gives off a spark when struck against steel. Found globally in nodules within carbonate rocks.
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