sedimentary
Chert Pebble
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, brown, or grey with waxy luster; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Tan, brown, or grey with waxy luster
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, brown, or grey with waxy luster; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within limestone or chalk beds. Often weathered out of host rock and rounded by water transport.
Uses & applications
Commonly used in construction aggregate, road surfacing, and historically used for making stone tools (arrowheads/scrapers) due to its sharp edges when broken.
Geological facts
Chert is nearly identical to flint; the names are often used interchangeably, though 'flint' usually refers to dark nodules found in chalk. It is highly resistant to chemical weathering, which is why it often survives as rounded pebbles in stream beds.
Field identification & locations
Identify in the field by its hardness (will scratch glass), waxy/dull luster, and smooth, rounded shape if found in water. Common in riverbeds, glacial tills, and gravel pits globally.
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