sedimentary

Chert (Iron-stained)

Microcrystalline silica (SiO2) with trace Iron oxide (Fe2O3)

Hardness: 6.5-7 on Mohs scale; Color: Cream, tan, and grey with rusty brown iron staining; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Hardness: 6
Color: Cream, tan, and grey with rusty brown iron staining
Luster: Dull to waxy

Identified March 25, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation from hydrothermal fluids. This specimen likely formed as nodules within carbonate rocks such as limestone or dolomite.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges. Modernly used as road aggregate, in landscaping, and occasionally as a gemstone (Jasper) if highly colored.

Geological facts

Chert is so hard it can scratch glass and steel. It was one of the most important minerals for early humans because its conchoidal fracture creates biological-grade sharp edges for cutting and piercing.

Field identification & locations

Identify in the field by its hardness (cannot be scratched by a knife), smooth waxy feel, and curved 'clamshell' fracture patterns. Common globally, often found in weathered limestone outcrops or river beds.

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