sedimentary

Chert (Tan/Grey)

Chert (Silicon Dioxide, SiO2)

Hardness: 7 on Mohs scale; Color: Tan, beige, or grey; Luster: Dull to waxy; Structure: Microcrystalline/cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Tan, beige, or grey
Luster: Dull to waxy

Identified March 30, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Tan, beige, or grey; Luster: Dull to waxy; Structure: Microcrystalline/cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from silica-rich fluids. Often found as nodules within limestone or chalk beds.

Uses & applications

Historically used for stone tools and weapons due to sharp edges when fractured. Modern uses include road surfacing, construction aggregate, and occasionally as a laboratory abrasive.

Geological facts

Chert is so hard it can scratch glass and steel. High-quality varieties like flint were essential to prehistoric survival for fire-starting and hunting. It is chemically identical to quartz but has a different microscopic structure.

Field identification & locations

Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch a steel knife blade. It feels very smooth and 'stony' compared to limestone. Commonly found in riverbeds or protruding from limestone cliffs.