sedimentary

Chert Nodules

Chert (Silicon Dioxide, SiO2)

Hardness: 7 on Mohs scale; Color: Cream, tan, or brown with a white cortex; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture).

Hardness: 7 on Mohs scale
Color: Cream, tan, or brown with a white cortex
Luster: Waxy to dull

Identified May 24, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Cream, tan, or brown with a white cortex; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture).

Formation & geological history

Formed by the precipitation of silica from groundwater within limestone or chalk beds. Replacing carbonate minerals with silica, these nodules often form in marine environments during the diagenesis of sediments.

Uses & applications

Historically used for stone tools due to sharp edges when fractured. Modern uses include road aggregate, decorative gravel, and occasionally lapidary work if the color is attractive.

Geological facts

Chert is essentially the same material as flint, but the term 'flint' is usually reserved for dark grey or black varieties found in chalk. It is highly resistant to chemical weathering.

Field identification & locations

Identified in the field by its distinct waxy texture on broken surfaces and its ability to scratch glass. It often features a rough, white outer rind called a 'cortex' which contrasts with the smoother interior.