sedimentary
Chert (and/or River Pebble)
Microcrystalline silica (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, brown, or buff; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Tan, brown, or buff
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, brown, or buff; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation from groundwater within limestone or chalk beds. Often smooth from water erosion in riverbeds.
Uses & applications
Historically used for stone tools (arrowheads). Modernly used as gravel for construction, road base, and decorative landscaping.
Geological facts
Chert is remarkably durable and resistant to chemical weathering. Varieties with high iron content are often called jasper, while dark varieties are known as flint.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture pattern. It is commonly found in river deposits, gravel pits, and sedimentary rock outcrops.
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