sedimentary
Chert (Field Stone)
Chert (SiO2)
Hardness: 7 (Mohs scale), Color: Brownish-grey with dark inclusions, Luster: Dull to waxy, Crystal Structure: Microcrystalline/Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Brownish-grey with dark inclusions, Luster: Dull to waxy, Crystal Structure: Microcrystalline/Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like radiolaria or diatoms) or through chemical precipitation in marine environments. Often found as nodules in limestone or chalk beds.
Uses & applications
Historically used for stone tools (arrowheads). Modernly used as road aggregate, in landscaping, or as a common decorative field stone.
Geological facts
Chert is extremely durable and chemically stable, which is why it often survives as a 'river pebble' or 'field stone' long after the surrounding rock has weathered away. Some varieties like Flint have been used by humans for hundreds of thousands of years.
Field identification & locations
Identify in the field by its hardness (will scratch glass), waxy feel when smooth, and sharp, shell-like (conchoidal) edges when broken. Commonly found in stream beds and garden soil.
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Cone-in-cone structure (Limestone)
Cone-in-cone structure (Secondary Sedimentary Structure)
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Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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Shale or Slaty Mudstone
Argillaceous sedimentary rock
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Sandstone
Arenite (primarily SiO2)
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