sedimentary
Chert (variously Flint)
Chert (Microcrystalline or cryptocrystalline Quartz, SiO₂)
Hardness: 7 (Mohs scale), Color: Dark gray to bluish-gray with tan/brown weathering, Luster: Waxy to dull, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: Dark gray to bluish-gray with tan/brown weathering, Luster: Waxy to dull, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by chemical precipitation in marine environments. Often found as nodules within limestone or chalk beds.
Uses & applications
Historically used for stone tools and fire-starting; industrial uses include silica sources for glass; construction aggregate; occasionally used in jewelry or as pocket stones.
Geological facts
Because it breaks with a conchoidal fracture, it creates edges sharper than a steel scalpel. Native Americans and early humans worldwide used it extensively for arrowheads and scrapers.
Field identification & locations
Identify in the field by high hardness (scratches glass), waxy appearance, and sharp, curved (conchoidal) edges when broken. Extremely common in stream beds and coastal cliffs.
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