sedimentary

Chert (Flint Nodules)

Silicon Dioxide (SiO2)

Hardness: 6.5-7 (Mohs scale); Color: Gray, brown, or tan with a white weathered cortex; Luster: Waxy to dull; Cut: Conchoidal fracture with sharp edges; Crystal Structure: Microcrystalline.

Hardness: 6
Color: Gray, brown, or tan with a white weathered cortex
Luster: Waxy to dull

Identified July 2, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 (Mohs scale); Color: Gray, brown, or tan with a white weathered cortex; Luster: Waxy to dull; Cut: Conchoidal fracture with sharp edges; Crystal Structure: Microcrystalline.

Formation & geological history

Formed through the precipitation of silica within sedimentary rock layers, often limestone or chalk. Many nodules began as silica-rich organic material like sponges or radiolarians that dissolved and re-precipitated in cavities.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) and as strike-a-lights for fire making. In modern times, used as a decorative stone in landscaping or as a coarse aggregate in construction.

Geological facts

Cromer Forest-bed flints and similar nodules were some of the first materials used by early humans for tool development. The white outer 'rind' is called a cortex and is caused by weathering and hydration of the surface silica.

Field identification & locations

Identify by its waxy interior when broken and its characteristic shell-like (conchoidal) fracture pattern. Commonly found in chalk downs, beaches, and riverbeds where sedimentary layers have eroded.