sedimentary
Chert (specifically a Chert Nodule or Flint)
Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: tan, grey, to dark brown with a white weathered cortex, Luster: waxy to dull, Structure: microcrystalline (no visible grains), Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, grey, to dark brown with a white weathered cortex, Luster: waxy to dull, Structure: microcrystalline (no visible grains), Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed by the accumulation of silica-rich organic or inorganic materials in sedimentary environments, often replacing calcium carbonate in limestone or chalk beds. Most are found in ancient marine deposits (Paleozoic to Mesozoic eras).
Uses & applications
Historically used for stone tools and fire-starting. Today used in road construction, landscape gravel, and occasionally as semi-precious gem material for cabochons.
Geological facts
Because it breaks with a conchoidal (shell-like) fracture, it produces incredibly sharp edges which were highly prized by early humans for making arrowheads, scrapers, and knives.
Field identification & locations
Identify in the field by its distinct waxy texture on fresh surfaces and its rounded, often knobby 'cortex' or skin. It will easily scratch glass but cannot be scratched by a steel nail. Common in riverbeds and limestone outcrops.
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