sedimentary
Chert / River Cobble
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, brown, and grey; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Tan, brown, and grey
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, brown, and grey; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed from the precipitation of silica-rich fluids within other sedimentary rocks or from the accumulation of siliceous biological remains. This specimen shows significant rounding from fluvial (river) transport and erosion.
Uses & applications
Historically used for stone tools (arrowheads); currently used as construction aggregate, road base, and common landscaping stone.
Geological facts
Chert was one of the most important minerals for early humans due to its ability to fracture with extremely sharp edges. It is often found as nodules in limestone or dolomite beds.
Field identification & locations
Identify by its extreme hardness (will scratch glass) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds and glacial till throughout North America.
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