sedimentary
Chert Nodule
Microcrystalline Silica (SiO2)
Hardness: 7 (Mohs scale), Color: Dark grey/brown with weathered tan exterior, 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: Dark grey/brown with weathered tan exterior, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed by the precipitation of silica within existing sediment layers, often replacing organic material or carbonate minerals. Commonly found in limestone or chalk beds. Period: Any, but frequent in Mesozoic and Paleozoic formations.
Uses & applications
Historically used for stone tools (flint knapping) and fire-starting. Currently used as a gemstone (jaspers), in construction aggregate, and for geological study.
Geological facts
Chert and its variety flint were among the first materials used by prehistoric humans to create sharp cutting tools. It is so hard that it can scratch steel and produces sparks when struck against iron-bearing metals.
Field identification & locations
Identify in the field by its characteristic smooth, curved (conchoidal) breakage patterns and its extreme hardness. It is often found as rounded nodules or layers within softer sedimentary rock like limestone.
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Sandstone or Siltstone
Arenite (if sandstone)
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Cone-in-cone structure (Limestone)
Cone-in-cone structure (Secondary Sedimentary Structure)
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Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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Shale or Slaty Mudstone
Argillaceous sedimentary rock
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Sandstone
Arenite (primarily SiO2)
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