sedimentary
Chert (Gravel)
Microcrystalline silica (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, or brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, or brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, or brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or by chemical precipitation in marine or groundwater environments. Can vary in age from Precambrian to Quaternary.
Uses & applications
Commonly used as construction aggregate, road base, and historically for stone tools (arrowheads) due to its sharp edges when broken.
Geological facts
Chert is so hard it can create sparks when struck against steel. It is one of the most common materials found in riverbeds and glacial till across the globe.
Field identification & locations
Identify in the field by its hardness (it should scratch glass) and its smooth, shell-like (conchoidal) breaking pattern. It is commonly found in gravel pits and driveways.
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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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Arenite (primarily SiO2)
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