sedimentary
Chert (or Jasper)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Brownish-red to earthy tones, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Brownish-red to earthy tones, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed primarily through the chemical precipitation of silica-rich fluids in ocean or lake environments, or by the replacement of limestone. It is common in Phanerozoic sedimentary sequences.
Uses & applications
Historically used for stone tools (knapping). Currently used for landscaping, road ballast, and lapidary work (jewelry and cabochons).
Geological facts
Chert was the primary material for prehistoric arrowheads and spear points because it breaks with a razor-sharp edge. Red varieties are often specifically called Jasper.
Field identification & locations
Identify in the field by its waxy appearance, extreme hardness (it will scratch glass), and its sharp, curved (conchoidal) edges when broken. It is extremely common in limestone regions.
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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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