sedimentary

Chert (Nodule)

Microcrystalline Silica (SiO2)

Hardness: 7 on Mohs scale; Color: Tan, brown, and grey; Luster: Waxy to dull; Structure: Cryptocrystalline/Amorphous; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Tan, brown, and grey
Luster: Waxy to dull

Identified April 6, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Tan, brown, and grey; Luster: Waxy to dull; Structure: Cryptocrystalline/Amorphous; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed through the precipitation of silica-rich fluids within sedimentary host rocks like limestone or chalk. Often accumulates from the remains of siliceous organisms like diatoms or radiolarians. May date from any geological era, commonly found in Cretaceous or Paleozoic bedding.

Uses & applications

Historically used for stone tools (arrowheads/scrapers) due to its sharp fracture. Today used as an abrasive, in construction aggregate, and as a decorative landscape stone.

Geological facts

Chert is so hard it can scratch steel. When struck against steel, it can produce sparks, leading to its historical use in flintlock firearms and fire-starting.

Field identification & locations

Identify in the field by its waxy texture and shell-like (conchoidal) curved breaks. Often found as rounded nodules or layers protruding from softer limestone outcrops.