sedimentary

Chert

Chert (Microcrystalline Silica, SiO2)

Hardness: 6.5–7 on Mohs scale; Color: typically grey, tan, or brown with distinct fine banding; Luster: dull to waxy; Structure: cryptocrystalline (microscopic quartz crystals); Cleavage: none, exhibits conchoidal (shell-like) fracture.

Hardness: 6
Color: typically grey, tan, or brown with distinct fine banding
Luster: dull to waxy

Identified March 22, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of silica-rich fluids in marine environments or the accumulation of silica-based micro-organisms like radiolaria or diatoms on the seafloor. This specimen displays parallel bedding/lamination indicative of sedimentary deposition.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; modernly used as a decorative road aggregate, in the manufacture of ceramics, or as a filter medium.

Geological facts

Chert is so hard it can strike sparks against steel, leading to its frequent historical confusion with flint (a dark variety of chert). It is often one of the most durable materials in old sedimentary rock sequences.

Field identification & locations

Identify in the field by its superior hardness (cannot be scratched by a steel knife) and its smooth, curved fracture surfaces. Commonly found as nodules or layers within limestone and dolostone formations.