sedimentary

Chert (and Flint nodule)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Gray, black, and tan/white cortex; Luster: Waxy to dull; Crystal Structure: Microcrystalline (Cryptocrystalline); Cleavage: None (has conchoidal fracture); Specific Gravity: 2.6-2.7

Hardness: 7 (Mohs scale)
Color: Gray, black, and tan/white cortex
Luster: Waxy to dull

Identified May 24, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation in marine environments. Often found as nodules within limestone or chalk beds.

Uses & applications

Historically used for stone tools and weapons (arrowheads/knives) due to sharp edges; used in construction aggregates and fire-starting kits (ferrocerium strikers).

Geological facts

Chert has a distinct conchoidal fracture, similar to glass, which allows it to be shaped into razor-sharp blades. It was one of the most important resources for prehistoric humans.

Field identification & locations

Identify in the field by its smooth, waxy surfaces on broken faces, sharp edges, and hardness (will scratch glass). Often found in limestone outcrops or as river cobbles. This specimen shows a weathered outer crust called a cortex.