sedimentary

Chert with Limestone Cortex

Silicon Dioxide (SiO2) with Calcium Carbonate (CaCO3)

AI-generated identification · may be incorrect

Hardness: 7 for chert, 3 for limestone; Color: Gray/brown core with white/cream outer layer; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).

Hardness: 7 for chert, 3 for limestone
Color: Gray/brown core with white/cream outer layer
Luster: Waxy to dull

Identified June 4, 2026More sedimentary →

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Physical properties

Hardness: 7 for chert, 3 for limestone; Color: Gray/brown core with white/cream outer layer; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) in marine environments. The white outer layer is the original limestone matrix or weathering rind.

Uses & applications

Historically used for stone tools and flintknapping. Currently used as decorative gravel, in road construction, and for geological study.

Geological facts

Chert is so hard it can scratch glass and steel. This specimen shows a classic 'nodule' appearance where the inner silica has replaced the surrounding limestone.

Field identification & locations

Identify by its 'noduled' appearance; the inner part is smooth and hard, while the outer part might react with acid. Frequently found in riverbeds or coastal cliffs containing limestone.