sedimentary
Chert (Nodule)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs), Color: Brown, tan, and gray mottled, Luster: Waxy to dull, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs), Color: Brown, tan, and gray mottled, Luster: Waxy to dull, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed by the accumulation of silica-rich organic remains (like diatoms or radiolaria) or chemical precipitation in marine environments, often replacing limestone or dolostone.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges; currently used as construction aggregate or decorative garden stone.
Geological facts
Chert is so hard it can strike sparks against steel to start fires. Variations in color are usually due to trace element impurities like iron or manganese oxides.
Field identification & locations
Identify by its extreme hardness, waxy texture, and conchoidal (shell-like) fracture patterns. It is very common in riverbeds and limestone outcrops worldwide.
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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
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Sandstone
Arenite (primarily SiO2)
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