sedimentary
Chert (Nodule)
Crystalline Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: Brownish-gray/drab, Luster: Waxy to dull, Crystal Structure: Microcrystalline/Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: Brownish-gray/drab, Luster: Waxy to dull, Crystal Structure: Microcrystalline/Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
Formation & geological history
Formed by the precipitation of silica from groundwater or within maritime sediments. Often found as nodules within limestone or chalk formations. These specimens range from Precambrian to recent ages.
Uses & applications
Historically used for making stone tools (arrowheads). Today used in road construction, as an abrasive, and sometimes for decorative lapidary work.
Geological facts
Because it breaks with a conchoidal (shell-like) fracture creating very sharp edges, it was one of the most important materials for early human survival for making knives and spear points.
Field identification & locations
Identifiable by its smooth, waxy texture and sharp, curved edges when broken. It will scratch steel and glass. Found globally in basins and riverbeds where sedimentary rocks have eroded.
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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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Shale or Slaty Mudstone
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Sandstone
Arenite (primarily SiO2)
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