sedimentary
Chert
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, or white; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, or white
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, or white; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed in marine environments through the accumulation of siliceous remains of organisms like diatoms and radiolarians, or through chemical precipitation in limestone or chalk beds. Commonly found in Paleozoic and Mesozoic sedimentary strata.
Uses & applications
Historically used for stone tools such as arrowheads due to its sharp edges when broken. Today used as road gravel, construction aggregate, and occasionally as a minor gemstone (jasper/agate varieties).
Geological facts
Chert is so hard it can scratch glass and sparks when struck against steel. It was one of the most vital materials for early humans to create survival tools.
Field identification & locations
Identify by its smooth, waxy feel and shell-like (conchoidal) fracture patterns. Frequently found in riverbeds or as nodules within limestone outcrops.
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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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