sedimentary
Chert (Flint Pebbles)
Microcrystalline quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, beige, or gray with a dull to waxy luster; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6
- Hardness
- 7 on Mohs scale
- Color
- Tan, beige, or gray with a dull to waxy luster
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, beige, or gray with a dull to waxy luster; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6
Formation & geological history
Formed as nodules in limestone or chalk beds from the precipitation of silica. This specific specimen shows signs of alluvial weathering and water-tumbling, likely from a river or beach environment.
Uses & applications
Historically used for making stone tools and fire-starting; currently used in construction aggregate, road surfacing, and as landscaping gravel.
Geological facts
Chert is so hard it can scratch glass and steel. It was one of the most important minerals for early humans because its conchoidal fracture creates extremely sharp edges for hunting and scraping.
Field identification & locations
Identify in the field by its smooth, rounded shape (if water-worn), extreme hardness, and distinctive shell-like (conchoidal) fracture pattern if broken. Commonly found in riverbeds and coastal areas.
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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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