sedimentary
Chert (often called Flint)
Chert (Microcrystalline Quartz, SiO2)
Hardness: 7 (Mohs scale), Color: tan, grey, or yellowish-brown, Luster: waxy to dull, Structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, grey, or yellowish-brown, Luster: waxy to dull, Structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich biogenic materials like radiolarian or diatom remains, or via chemical precipitation in carbonate rock. Found in nodules within limestone or chalk beds.
Uses & applications
Used historically for stone tools and starting fires; currently used in road construction, landscaping, and occasionally as semi-precious gemstones when colorful (Jasper).
Geological facts
Because it breaks with a conchoidal fracture like glass, it creates edges sharper than a surgeon's scalpel. Flint is a specific dark variety of chert often found in chalk deposits.
Field identification & locations
Identify by its waxy appearance and smooth, curved (conchoidal) breaks. It is extremely hard and will scratch glass. Look for it along riverbeds or embedded in limestone cliffs.
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