sedimentary
Chert (and Cherty Limestone)
Silicon Dioxide (SiO2) with impurities
Hardness: 7 (Mohs scale), Color: tan, buff, or brown, Luster: dull to waxy, Crystal Structure: Cryptocrystalline, Fracture: Conchoidal to irregular, Specific Gravity: 2.5-2.8
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, buff, or brown, Luster: dull to waxy, Crystal Structure: Cryptocrystalline, Fracture: Conchoidal to irregular, Specific Gravity: 2.5-2.8
Formation & geological history
Formed through the accumulation of silica-rich microfossils (like radiolarians or diatoms) on the ocean floor or by the precipitation of silica from groundwater within limestone/chalk beds. Often Paleozoic or Mesozoic in age.
Uses & applications
Historically used for stone tools (arrowheads). Modern uses include road aggregate, gravel, and occasionally for lapidary work if highly colorful.
Geological facts
Chert is so hard it can spark when struck against steel. It is nearly identical in composition to flint, though 'flint' usually refers to dark nodules found specifically in chalk.
Field identification & locations
Look for a microcrystalline texture that doesn't show individual grains under a hand lens. It usually scratches glass easily and may show curved (conchoidal) breaks. Common in river beds and limestone outcrops.
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