sedimentary

Chert Nodule (with Flint core)

Sedimentary Cryptocrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Tan to cream exterior with a darker brown/honey-colored interior core; Luster: Dull to waxy; Structure: Cryptocrystalline/microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 on Mohs scale
Luster: Dull to waxy

Identified April 22, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of silica within limestone or chalk beds. This typically occurs in marine environments where silica-rich fluids replace carbonate material. Many nodules date back to the Cretaceous or Jurassic periods.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) and strike-a-lights due to sparks; currently used in construction aggregate and as decorative garden stones.

Geological facts

This specimen shows a distinct 'rind' or cortex on the outside with a visible darker interior core revealed by a break. It is often referred to as 'Hag Stones' if they have natural holes, or commonly just flint when found in chalk.

Field identification & locations

Identify in the field by its smooth, water-worn exterior and its characteristic conchoidal (shell-like) fracture pattern when broken. It is much harder than the limestone it is often found within.

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