sedimentary

Chert (Nodule)

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: typically white, grey, or tan; Luster: dull to waxy; Structure: Microcrystalline/cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.5–2.8

Hardness: 7 on Mohs scale
Color: typically white, grey, or tan
Luster: dull to waxy

Identified March 7, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: typically white, grey, or tan; Luster: dull to waxy; Structure: Microcrystalline/cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.5–2.8

Formation & geological history

Formed by the precipitation of silica in sedimentary rock layers, often replacing organic material or filling voids. They typically form in marine environments during the late Paleozoic to Mesozoic eras.

Uses & applications

Historically used for stone tools/arrowheads due to its sharp edges; modern uses include road surfacing, decorative landscaping, and as an ingredient in ceramics and glass.

Geological facts

Chert nodules are often found within limestone or chalk beds. Because of their hardness and chemical stability, they remain behind as 'residual gravel' after the surrounding limestone has weathered away.

Field identification & locations

Identify in the field by its smooth, waxy texture and inability to be scratched by a steel knife. It is frequently found in riverbeds or eroded limestone cliffs across the Midwestern and Southeastern United States.