sedimentary
Chert (often called Flint when dark)
Microcrystalline silicon dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: tan, yellow, Gray, Luster: waxy to dull, Crystal structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, yellow, Gray, Luster: waxy to dull, Crystal structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed by the accumulation of siliceous remains of organisms (like diatoms or radiolarians) or chemical precipitation from silica-rich fluids. Often found as nodules or layers within limestone and chalk beds.
Uses & applications
Historically used for stone tools and arrowheads; currently used in construction aggregate, road surfacing, and as a component in ceramics or glass-making.
Geological facts
Because it breaks with a conchoidal fracture (producing razor-sharp edges), it was the most important material for prehistoric tool-making. It is very resistant to chemical weathering.
Field identification & locations
Identify in the field by its waxy appearance, extreme hardness (scratches glass), and shell-like (conchoidal) curved fractures. Common globally in limestone-rich regions like the Midwestern USA or the chalk cliffs of Europe.
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