sedimentary
Chert (and/or Flint)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: light gray to bluish-gray with waxy luster, Crystal structure: microcrystalline/cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: light gray to bluish-gray with waxy luster, Crystal structure: microcrystalline/cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed as nodules or layered deposits in limestone or chalk beds. It occurs when silica-rich fluids replace carbonate minerals or accumulate from microscopic organisms like radiolarians or diatoms. Most specimens date from the Phanerozoic eon.
Uses & applications
Historically used for stone tools (knapping) and fire-starting. Currently used in road construction, as a flux in glass manufacturing, and semi-precious jewelry (lapidary).
Geological facts
Chert has a conchoidal fracture, allowing it to break into razor-sharp edges. Flint is a specific variety of chert often associated with chalk or marly limestone.
Field identification & locations
Identify in the field by its sharp, curved edges when broken, waxy feel, and its ability to scratch glass. It will often produce sparks when struck against steel. Common in limestone outcrops.
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