sedimentary
Chert (River Pebble)
Microcrystalline Quartz (SiO2)
Hardness: 6.5–7 on the Mohs scale. Color: Brown, tan, or grey. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.65.
- Hardness
- 6
- Color
- Brown, tan, or grey
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5–7 on the Mohs scale. Color: Brown, tan, or grey. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.65.
Formation & geological history
Formed from the recrystallization of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation from groundwater within sedimentary beds. This specimen is water-worn, smoothed by fluvial erosion.
Uses & applications
Used historically for stone tools (arrowheads). In modern times, used as decorative aggregate in landscaping, construction gravel, or as tumbled 'worry stones' for collectors.
Geological facts
Chert is so tough and chemically stable that it often survives long after the surrounding rock has weathered away. Known as 'flint' when found in chalk or marly limestone.
Field identification & locations
Identify by its waxy texture, hardness (it will scratch glass), and the way it breaks into smooth, curved surfaces. Commonly found in riverbeds, glacial tills, and beaches.
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