sedimentary
Chert (Pebble)
Chert (Microcrystalline Quartz, SiO2)
Hardness: 7 on Mohs scale; Color: Grayish-blue to dark grey; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Grayish-blue to dark grey
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Grayish-blue to dark grey; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; 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 within sedimentary rock layers, often found in limestone or chalk beds.
Uses & applications
Historically used for stone tools/arrowheads due to its sharp edges when broken; used modernly in road construction, as decorative gravel, and occasionally for high-durability laboratory equipment parts.
Geological facts
Chert is so hard it can strike sparks from steel, which led to its subtype 'flint' being used in early firearms and fire-starting kits. It is exceptionally resistant to weathering.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture pattern. Commonly found in riverbeds or coastal areas as weathered pebbles.
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