sedimentary
Chert (Smooth River Pebble)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, brown with mottled patterns; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, brown with mottled patterns
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, brown with mottled patterns; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation from groundwater. This specimen has been rounded and smoothed by fluvial (river) action over thousands of years.
Uses & applications
Historically used for stone tools (arrowheads); currently used as decorative landscaping stone, in construction aggregate, and occasionally for lapidary tumbling.
Geological facts
Chert is so hard it can strike sparks against steel. It is one of the most common materials used by prehistoric humans for tool-making because of its predictable conchoidal fracturing.
Field identification & locations
Identify in the field by its waxy texture, inability to be scratched by a steel knife, and smooth, rounded water-worn exterior. Found worldwide in riverbeds, glacial tills, and sedimentary basins.
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