sedimentary
Chert Nodules (with Quartz Inclusions)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs), Color: Grey, tan, and white, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs), Color: Grey, tan, and white, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed by the precipitation of silica within sedimentary rock layers, often replacing carbonate material like limestone or chalk. Commonly found in Paleozoic to Mesozoic limestone formations.
Uses & applications
Historically used for making stone tools (flint knapping). Today used as decorative gravel, in landscaping, and as a raw material in some ceramic glazes.
Geological facts
Chert nodules often contain micro-fossils and are highly resistant to weathering, which is why they are frequently found in riverbeds and on beaches after the surrounding limestone has eroded away.
Field identification & locations
Identify in the field by its hardness (scratches glass), waxy texture, and smooth, curved (conchoidal) surface where broken. Common in creek beds and road cuts through sedimentary regions.
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