sedimentary

Chert (Nodule)

Silicon Dioxide (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 (Mohs scale); Color: Brown, tan, gray; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Brown, tan, gray
Luster: Dull to waxy

Identified March 14, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Brown, tan, gray; Luster: Dull to waxy; Structure: Microcrystalline; 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 in marine or groundwater environments. These nodules often displace limestone or chalk over millions of years.

Uses & applications

Historically used for stone tools (flintknapping); currently used in road construction, as an abrasive, and small polished pieces are used in decorative gravel or folk art.

Geological facts

Chert is so hard it can strike sparks against steel. It was the primary material for prehistoric arrowheads and spear tips because of its predictable, razor-sharp fracturing pattern.

Field identification & locations

Identify in the field by its extreme hardness (scratches glass) and smooth, shell-like (conchoidal) fracture surfaces when broken. Common in limestone outcrops and riverbeds. Collectors look for unique shapes or 'geode' interiors.