sedimentary
Chert (River Pebble)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: typically dark brown, grey, or black; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- typically dark brown, grey, or black
- Luster
- waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically dark brown, grey, or black; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation from sea water. This specific specimen has been smoothed by water transport in a river or coastal environment.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp fracture; currently used in construction aggregate, landscaping, and sometimes as a polishing medium.
Geological facts
Chert is extremely durable and chemically stable, allowing it to survive long-distance transport in rivers. It has been used by humans for hundreds of thousands of years to create fire by striking it against steel or pyrite.
Field identification & locations
Identify in the field by its hardness (will scratch glass), smooth waxy texture when wet, and its characteristic curved (conchoidal) shell-like breaks. It is ubiquitous in gravel bars and riverbeds worldwide.
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