sedimentary
Chert (specifically Gray Chert)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: light to dark gray/tan, Luster: waxy to dull, Crystal structure: microcrystalline/cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: light to dark gray/tan, Luster: waxy to dull, Crystal structure: microcrystalline/cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed from the accumulation of siliceous remains of marine organisms (like diatoms or radiolarians) or through chemical precipitation in groundwater. Often occurs as nodules or layers within limestone or shale.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges; used today as a decorative stone, in road construction, and as flint for fire-starting.
Geological facts
Chert has been used by humans for over 2 million years. When it is distinctively colorful, it is often called Jasper; when it is dark and high in organic matter, it is called Flint.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in limestone outcrops, river beds, and gravel pits.
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