sedimentary
Chert (Flint)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, black, or tan; Luster: Dull to waxy; Structure: Cryptocrystalline (no visible grains); Cleavage: None (exhibits conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, black, or tan
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, black, or tan; Luster: Dull to waxy; Structure: Cryptocrystalline (no visible grains); Cleavage: None (exhibits conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the precipitation of silica from groundwater or the accumulation of siliceous skeletal remains (like diatoms or radiolarians) in marine environments. It is often found as nodules in limestone or chalk beds.
Uses & applications
Historically used for stone tools and starting fires; currently used in construction aggregate, road surfacing, and as a semi-precious gemstone for decorative carvings or jewelry.
Geological facts
Because it breaks with a conchoidal 'shell-like' fracture, it produces extremely sharp edges, making it the primary material for prehistoric arrowheads, scrapers, and knives.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, curved fracture surfaces. It is ubiquitous in sedimentary basins worldwide, particularly where limestone is present.
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