sedimentary
Chert (often called Flint)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: tan, brown, or gray with white patches, Luster: Dull to waxy, Crystal structure: Trigonal (microscopic), Cleavage: None (conchoidal fracture), Specific gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, brown, or gray with white patches, Luster: Dull to waxy, Crystal structure: Trigonal (microscopic), Cleavage: None (conchoidal fracture), Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation from silica-rich fluids within other sedimentary rocks like limestone. Common in Paleozoic to Cenozoic strata.
Uses & applications
Historically used for stone tools (arrowheads). Today used in road construction, as an abrasive, and occasionally in lapidary work or for starting fires (flint and steel).
Geological facts
Because of its conchoidal fracture, it breaks into razor-sharp edges, making it the primary material for prehistoric survival tools globally. It is extremely resistant to chemical weathering.
Field identification & locations
Identify by its 'glassy' but opaque look and its tendency to break in shell-like curves (conchoidal fracture). It will easily scratch glass and cannot be scratched by a steel knife. Commonly found as nodules or layers in limestone beds.
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