sedimentary
Chert (with Limestone cortex)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: tan, grey, cream with a white chalky exterior (cortex), Luster: waxy to dull, Structure: microcrystalline quartz, Cleavage: none (conchoidal fracture)
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, grey, cream with a white chalky exterior (cortex), Luster: waxy to dull, Structure: microcrystalline quartz, Cleavage: none (conchoidal fracture)
Formation & geological history
Formed by the accumulation of silica-rich fluids (often from marine organisms like sponges or diatoms) within limestone or chalk beds. Commonly found in Paleozoic or Mesozoic sedimentary basins.
Uses & applications
Historically used for making stone tools and fire-starting flint; currently used for road beds and gravel, or as decor in its more colorful agate-like forms.
Geological facts
Chert is so durable that it often remains as pebbles in stream beds long after the surrounding limestone has eroded away. It breaks with extremely sharp edges.
Field identification & locations
Identify it by its smooth, waxy interior surface often hidden by a chalky white skin. Collectors should look for its distinctive shell-like (conchoidal) fracture pattern when broken.
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