sedimentary
Chert (Nodule)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: tan, grey, brown, Luster: waxy to dull, Structure: microcrystalline/cryptocrystalline, Fracture: conchoidal, Cleavage: none
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, grey, brown, Luster: waxy to dull, Structure: microcrystalline/cryptocrystalline, Fracture: conchoidal, Cleavage: none
Formation & geological history
Formed through the biochemical precipitation of silica from seawater or the replacement of limestone/chalk by silica-rich fluids. Common in Precambrian to Cenozoic sedimentary layers.
Uses & applications
Historically used for making stone tools (arrowheads). Modernly used as road aggregate, in jewelry (when decorative), and as flint for spark-striking.
Geological facts
Chert is so hard it can scratch steel. It was the primary material for human tools for millions of years. Varieties include flint, jasper, and agate.
Field identification & locations
Identify by its waxy texture, conchoidal (shell-like) fracture pattern when broken, and extreme hardness. Often found as rounded nodules within limestone or chalk beds.
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