sedimentary
Chert (and Flint)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: Dark grey, tan, and earthy tones, Luster: Dull to waxy, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Dark grey, tan, and earthy tones, Luster: Dull to waxy, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich microfossils like diatoms or radiolarians, or by chemical precipitation in marine or groundwater environments. Often found as nodules in limestone or chalk beds.
Uses & applications
Historically used for stone tools and starting fires; currently used in construction aggregate, semi-precious jewelry, and as a decorative specimen.
Geological facts
Chert has been used by humans for hundreds of thousands of years to create sharp-edged tools like arrowheads. When struck against steel, it creates sparks due to its hardness and composition.
Field identification & locations
Identify by its waxy luster, extreme hardness (scratches glass), and distinctive shell-like (conchoidal) curved fracture lines when broken. Commonly found in riverbeds or within limestone outcrops.
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Cone-in-cone structure (Secondary Sedimentary Structure)
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Clastic Sedimentary Rock Grain (SiO2 dominated)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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Sandstone
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