sedimentary
Chert (River Pebble)
Microcrystalline Quartz (SiO2)
Hardness: 7 Mohs scale; Color: Brown, tan, reddish-brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 Mohs scale
- Color
- Brown, tan, reddish-brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 Mohs scale; Color: Brown, tan, reddish-brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed via the accumulation of silica-rich organic remains or chemical precipitation in sedimentary basins. This specimen shows significant rounding and smoothing due to fluvial (river) erosion over thousands of years.
Uses & applications
Historically used for making stone tools due to its conchoidal fracture. Currently used in road construction, landscaping, and occasionally as semi-precious tumble-polished stones for jewelry.
Geological facts
The brown color is often caused by iron oxide (limonite) inclusions. Chert is one of the most durable rocks, often surviving long-distance transport in rivers and along beaches long after other rocks have eroded away.
Field identification & locations
Identified by its high hardness (cannot be scratched by a steel knife), smooth waxy texture, and unique curved 'conchoidal' fracture marks. Commonly found in riverbeds, gravel pits, and glacial till.
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