sedimentary
Chert (River Pebble)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Dark brown, grey, or black with waxy luster when wet; Structure: Cryptocrystalline; Cleavage: None, exhibits conchoidal (shell-like) fracture; Specific Gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Dark brown, grey, or black with waxy luster when wet
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Physical properties
Hardness: 7 (Mohs scale); Color: Dark brown, grey, or black with waxy luster when wet; Structure: Cryptocrystalline; Cleavage: None, exhibits conchoidal (shell-like) fracture; Specific Gravity: 2.65.
Formation & geological history
Formed via the accumulation of siliceous remains of marine organisms (like radiolaria or diatoms) or chemical precipitation from silica-rich fluids in sedimentary basins. This specimen has been rounded and smoothed by fluvial (river) transport over thousands of years.
Uses & applications
Historically used for making stone tools (flintknapping) due to its sharp edges; used today as decorative garden stone, river rock for landscaping, and road aggregate.
Geological facts
Chert is so hard it can scratch glass and steel. High-quality varieties like flint were essential for starting fires and weaponry in the Stone Age. It is often found as nodules inside limestone beds.
Field identification & locations
Identify in the field by its smoothness, extreme hardness, and conchoidal fracture patterns if chipped. Common in riverbeds, glacial tills, and coastal beaches. Collectors should look for unique banding or fossils within.
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