sedimentary
Chert (Chalcedony nodule)
Silicon Dioxide (SiO2)
Hardness: 6.5-7 (Mohs scale); Color: Tan, yellow-brown, or buff; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6
- Hardness
- 6
- Color
- Tan, yellow-brown, or buff
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs scale); Color: Tan, yellow-brown, or buff; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6
Formation & geological history
Formed by the accumulation of siliceous remains of organisms (like diatoms or radiolarians) or by the precipitation of silica from groundwater within sedimentary rock layers like limestone or chalk.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges when broken. Today used in tumbling, decorative gravel, and as a component in cement.
Geological facts
Chert is extremely durable and weather-resistant, which is why it often remains as pebbles in stream beds long after the surrounding limestone has eroded away. Dark varieties are often called flint.
Field identification & locations
Identify by its waxy texture and conchoidal (shell-like) fracture patterns. It is much harder than a nail and will scratch glass. Look for it in limestone outcrops or as rounded pebbles in rivers worldwide.
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