sedimentary
Chert
Chert (SiO2)
Hardness: 7 (Mohs scale), Color: White to light gray with waxy luster, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: White to light gray with waxy luster, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.65
Formation & geological history
Formed primarily through the chemical or biological precipitation of silica from sea water or from the replacement of other minerals. Often found as nodules in limestone or chalk formations.
Uses & applications
Historically used for making stone tools (arrowheads). Today used in road surfacing, as an abrasive, and occasionally for decorative lapidary work.
Geological facts
Many Native American tribes highly valued specific outcrops of high-quality chert for trade and tool manufacturing. It is incredibly chemically stable and resistant to weathering.
Field identification & locations
Identify by its characteristic conchoidal (shell-like) fracture pattern and its ability to scratch glass. Commonly found in sedimentary basins or as rounded pebbles in riverbeds.
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