sedimentary

Chert (Nodule)

Chert (Silicon Dioxide, SiO2)

Hardness: 7 on Mohs scale; Color: Cream, tan, and rusty brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6

Hardness: 7 on Mohs scale
Color: Cream, tan, and rusty brown
Luster: Waxy to dull

Identified June 13, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Cream, tan, and rusty brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6

Formation & geological history

Formed via the accumulation of silica-rich organic remains (like diatoms or radiolaria) or chemical precipitation within limestone or chalk beds. Commonly Phanerozoic in age.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp fracture edges. Today used in construction aggregate and as a collector's specimen.

Geological facts

Chert is so hard it can strike sparks against steel, which made it essential for flintlock firearms. It is chemically identical to quartz but has a different microscopic structure.

Field identification & locations

Identified in the field by its extreme hardness (cannot be scratched by a steel knife) and waxy, smooth fracture surfaces. Often found as rounded nodules in sedimentary outcrops.