sedimentary

Chert (Flint Nodules)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale); Color: Gray, black, and tan (shown with a typical weathered white chalky cortex); Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 (Mohs scale)
Luster: Waxy to dull

Identified March 20, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the chemical precipitation of silica-rich fluids within sedimentary rock layers, often limestone or chalk. Many nodules began as organic matter (like sponges) that was replaced by silica over millions of years.

Uses & applications

Historically used for stone tools and weapons due to sharp edges; currently used in construction aggregate, as fire-starting flints, and occasionally as decorative landscaping stone.

Geological facts

Flint was one of the most important minerals for early human civilization, providing the sharpest edges for hunting and the ability to create sparks for fire. The white outer layer is called a 'cortex' and is formed by the leaching of silica or the presence of impurities.

Field identification & locations

Identify by its 'shell-like' conchoidal fracture patterns and ability to scratch glass. Commonly found in riverbeds, chalk cliffs (like the White Cliffs of Dover), and gravel pits. Collectors look for pieces with unique banding or translucent edges.

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