sedimentary
Chert (specifically White Chert or Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs), Color: Cream, white, or light tan, Luster: Waxy to vitreous, Crystal Structure: Microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs), Color: Cream, white, or light tan, Luster: Waxy to vitreous, Crystal Structure: Microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation from silica-rich fluids within other sedimentary rocks like limestone. Common in Paleozoic to Mesozoic strata.
Uses & applications
Historically used for stone tools (arrowheads). Today used as a decorative stone, road surfacing, and sometimes in porcelain manufacturing or as a gemstone in the form of Jasper/Agate.
Geological facts
Chert produces sparks when struck against steel, which made it a critical survival tool for making fire throughout human history. It is one of the most durable sedimentary rocks due to its high silica content.
Field identification & locations
Identify in the field by its very smooth, waxy surface and its sharp, shell-like (conchoidal) fracture edges. It cannot be scratched by a steel knife. Commonly found in riverbeds or as nodules within limestone outcrops.
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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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