sedimentary
Chert (specifically Dark Chert or Flint)
Silicon dioxide (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Dark grey to black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 6
- Color
- Dark grey to black
- Luster
- Waxy to dull
Identified More sedimentary →
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Dark grey to black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed by the accumulation of microcrystalline silica, often from the skeletal remains of silica-secreting organisms (like diatoms or radiolaria) in deep-sea environments, or through chemical precipitation in limestone or chalk beds.
Uses & applications
Historically used for stone tools and weapons due to its sharp edges when fractured. Modernly used in road construction, as a component of concrete, and sometimes as a specimen for geological study.
Geological facts
Dark-colored chert is often called flint. It was one of the most important minerals for prehistoric humans because it produces very sharp edges and can generate sparks for fire when struck against steel.
Field identification & locations
Identify it by its waxy luster, extreme hardness (it will scratch glass), and conchoidal (shell-like) fracture patterns. Frequently found in riverbeds, beaches, or as nodules in limestone outcrops.
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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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