sedimentary
Chert (or Chalcedony)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale. Color: Gray, tan, and brown with reddish oxidation. Luster: Waxy to dull. Crystal Structure: Hexagonal (cryptocrystalline). Cleavage: None, exhibits conchoidal fracture. Specific Gravity: 2.65.
- Hardness
- 6
- Color
- Gray, tan, and brown with reddish oxidation
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale. Color: Gray, tan, and brown with reddish oxidation. Luster: Waxy to dull. Crystal Structure: Hexagonal (cryptocrystalline). Cleavage: None, exhibits conchoidal fracture. Specific Gravity: 2.65.
Formation & geological history
Formed via the accumulation of siliceous remains of marine organisms (like diatoms or radiolaria) or through chemical precipitation from silica-rich fluids in sedimentary basins. Primarily found in Phanerozoic rock sequences.
Uses & applications
Historically used for stone tools and arrowheads due to its sharp edges. Currently used as construction aggregate, in decorative landscaping, and occasionally for lapidary work.
Geological facts
Because it is so hard and chemically stable, chert often survives as rounded river pebbles long after the surrounding rock has eroded away. It is one of the most common materials used for prehistoric tools.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds and limestone outcrops.
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