sedimentary
Chert (Pebble)
Microcrystalline to Cryptocrystalline Quartz (SiO2)
Hardness: 7 Mohs; Color: Gray, tan, or brown; Luster: Waxy to dull; Structure: Non-crystalline mass (amorphous appearance); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 Mohs
- Color
- Gray, tan, or brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 Mohs; Color: Gray, tan, or brown; Luster: Waxy to dull; Structure: Non-crystalline mass (amorphous appearance); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like diatoms or radiolaria) or chemical precipitation from groundwater within sedimentary beds. These specimens are often found as weathered pebbles in secondary deposits.
Uses & applications
Primarily used historically for stone tools and weapons due to its sharp edges; currently used for road ballast, landscaping, and as a raw material for silicon production.
Geological facts
Chert is a very hard and durable rock that can spark when struck against steel. Flint is a high-quality variety of chert often associated with chalk or limestone.
Field identification & locations
Identify by its extreme hardness (scratches glass), waxy luster when freshly broken, and smooth, curved conchoidal fractures. Common in riverbeds and glacial tills worldwide.
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