sedimentary
Chert (varieties: Flint or Chalcedony)
Microcrystalline silica (SiO2)
Hardness: 7 (Mohs scale). Color: Grey, tan, or brown with a waxy luster. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Grey, tan, or brown with a waxy luster
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Physical properties
Hardness: 7 (Mohs scale). Color: Grey, tan, or brown with a waxy luster. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of siliceous remains (like diatoms or radiolarians) or chemical precipitation in sedimentary environments. Common in marine limestone and volcanic ash beds.
Uses & applications
Used historically for stone tools and fire-starting. Industrially used as an abrasive, in road construction, and higher-quality translucent pieces are used in jewelry as semi-precious stones.
Geological facts
Chert was the primary material used by prehistoric humans to create spearheads and arrowheads due to its conchoidal fracture which creates razor-sharp edges.
Field identification & locations
Identify by its waxy appearance and sharp, curved (conchoidal) breakage patterns. Found globally in nodules within limestone or as bedded deposits. It will scratch glass easily.
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