sedimentary

Chert (Nodular Chert)

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Brown, grey, tan, or black with waxy/dull luster; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Brown, grey, tan, or black with waxy/dull luster

Identified April 8, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Brown, grey, tan, or black with waxy/dull luster; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Formation & geological history

Formed through the accumulation of silica-rich organic debris (like radiolarians or sponge spicules) or by the chemical replacement of limestone/chalk by silica-rich groundwater. Often found in nodules within sedimentary layers.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; used today as a component in construction aggregate and occasionally as a decorative landscape stone.

Geological facts

Chert is extremely durable and chemically stable. Large concentrations are often found in the Mississippian and Pennsylvanian age rock layers in North America. Flint is a high-quality variety of dark chert.

Field identification & locations

Look for a smooth, waxy texture and curved, glass-like (conchoidal) fractures when broken. It is harder than steel and will scratch glass. Frequently found in riverbeds or eroded out of limestone cliffs.