sedimentary

Chert (Nodule)

Microcrystalline Silica (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, brown, or tan; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Gray, brown, or tan
Luster: Waxy to dull

Identified April 11, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Gray, brown, or tan; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & geological history

Formed through the accumulation of silica-rich biogenic remains (like diatoms or radiolarians) or chemical precipitation in marine environments. Often found as nodules within limestone or chalk beds.

Uses & applications

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

Geological facts

Chert is so hard it can strike sparks against steel. It is one of the most common materials used by prehistoric humans for tool-making because of its predictable conchoidal fracturing.

Field identification & locations

Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like fracture surfaces. It often feels denser and smoother than the surrounding limestone matrix.