sedimentary
Chert (River Pebble)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray with yellow-brown iron oxide staining; Luster: Waxy to vitreous when polished; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).
- Hardness
- 7 on Mohs scale
- Color
- Gray with yellow-brown iron oxide staining
- Luster
- Waxy to vitreous when polished
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray with yellow-brown iron oxide staining; Luster: Waxy to vitreous when polished; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed via the accumulation of siliceous remains of microscopic organisms (like diatoms or radiolarians) or chemical precipitation from silica-rich fluids in sedimentary basins. This specimen has been rounded by fluvial (river) erosion.
Uses & applications
Historically used for stone tools due to its sharp edges. Modern uses include aquarium gravel, decorative landscaping, and as a tumbling media for rock collectors.
Geological facts
Chert was one of the most vital materials for early humans because its conchoidal fracture allows it to be worked into arrowheads and knives. High-quality versions are often called flint.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and smooth, glass-like texture. Commonly found in riverbeds, glacial tills, and limestone outcrops globally.
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