sedimentary
Chert (Fossiliferous)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: Tan to brown, Luster: Dull to waxy, Crystal structure: Microcrystalline (cryptocrystalline), Fracture: Conchoidal, Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Tan to brown, Luster: Dull to waxy, Crystal structure: Microcrystalline (cryptocrystalline), Fracture: Conchoidal, Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or sponges) on the seafloor, often replacing original limestone through chemical precipitation. Frequently found in Paleozoic or Mesozoic strata.
Uses & applications
Historically used for stone tools (arrowheads); currently used as road aggregate, in landscaping, and as a collector's specimen if fossils are present.
Geological facts
Chert is so hard it can strike sparks from steel. Specimens like this often contain micro-fossils of ancient marine life such as crinoids, shells, or bryozoans.
Field identification & locations
Identify in the field by its hardness (cannot be scratched by a steel blade) and its smooth, shell-like fracture surfaces. Look for it in limestone basins or river gravels.
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