sedimentary

Chert Nodule (with Ironstone Cortex)

Microcrystalline Silica (SiO2)

Hardness: 7 (Mohs scale). Color: Grayish-blue core with a tan/brown outer cortex. Luster: Dull to waxy. Crystal Structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.6-2.7.

Hardness: 7 (Mohs scale)
Color: Grayish-blue core with a tan/brown outer cortex
Luster: Dull to waxy

Identified May 9, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the replacement of limestone or other carbonate rocks by silica-rich fluids. The silica precipitates into nodules or concretions during diagenesis in marine environments. The outer layer represents a weathered iron-rich cortex.

Uses & applications

Historically used for stone tools (flintknapping) due to its sharp edges; used today as a decorative rock, in road construction (aggregate), and sometimes for fire-striking.

Geological facts

Chert nodules are often called 'naturals' and can sometimes contain tiny microfossils. High-quality varieties are known as flint and have been essential to human technology for millions of years.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture pattern when broken. Commonly found in limestone outcrops or as rounded cobbles in stream beds.

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