sedimentary

Chert (with Iron Staining)

Chert (SiO2)

Hardness: 7 (Mohs scale), Color: tan, white, brown with red/orange iron oxidation, Luster: waxy to dull, Crystal structure: microcrystalline/cryptocrystalline, Cleavage: none (conchoidal fracture).

Identified May 31, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale), Color: tan, white, brown with red/orange iron oxidation, Luster: waxy to dull, Crystal structure: microcrystalline/cryptocrystalline, Cleavage: none (conchoidal fracture).

Formation & geological history

Formed by the accumulation of siliceous remains of microscopic organisms (like diatoms or radiolarians) or through the chemical precipitation of silica in groundwater. Often found as nodules in limestone or chalk beds.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges. Currently used in road construction, as an abrasive, and occasionally for lapidary work.

Geological facts

Chert is essentially the same material as flint, but 'flint' is often reserved for specimens found in chalk. It breaks with a conchoidal fracture, creating edges sharper than a steel razor.

Field identification & locations

Identify by its high hardness (scratches glass), waxy appearance on fresh surfaces, and curved, shell-like (conchoidal) fracture patterns. Frequently found in creek beds and limestone outcrops.