sedimentary

Chert (Gravel)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Tan, brown, gray, or white; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Tan, brown, gray, or white
Luster: Waxy to dull

Identified April 3, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Tan, brown, gray, or white; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation of silica in marine environments. Often found as nodules in limestone or as distinct bedded layers.

Uses & applications

Used primarily in construction as road base, gravel, and concrete aggregate. Historically used for stone tools (flintknapping) and fire-starting.

Geological facts

Chert is so hard it can scratch glass and sparks when struck against steel. It is one of the most common materials used by prehistoric humans to make arrowheads and scrapers.

Field identification & locations

Identify in the field by its very hard, smooth waxy surface and its characteristic 'shell-like' (conchoidal) curved breaks. Commonly found in riverbeds and as landscape gravel.