sedimentary
Chert (specifically Black Chert or Flint)
Silicon Dioxide (SiO2) - Microcrystalline Quartz
Hardness: 7 (Mohs scale); Color: Black to dark grey; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline (hexagonal system); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 (Mohs scale)
- Color
- Black to dark grey
- Luster
- Waxy to vitreous
Identified More sedimentary →
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Physical properties
Hardness: 7 (Mohs scale); Color: Black to dark grey; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline (hexagonal system); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation from silica-rich fluids in sedimentary basins. Primarily found in limestone or chalk beds.
Uses & applications
Used historically for stone tools and weapons (arrowheads, scrapers); currently used in road construction, as jewelry (when patterned), and occasionally in start-fire kits.
Geological facts
Chert was one of the most important raw materials for early humans because of its ability to be knapped into sharp edges. Flint is a specific variety of chert often found in chalk.
Field identification & locations
Identify in the field by its characteristic shell-like (conchoidal) fracture pattern and its ability to scratch glass. It is commonly found in nodules within limestone or as pebbles in stream beds.
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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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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
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Shale or Slaty Mudstone
Argillaceous sedimentary rock
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Sandstone
Arenite (primarily SiO2)
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