sedimentary
Chert (and Flint)
Silicon dioxide (SiO2) - Microcrystalline or Cryptocrystalline Quartz
Hardness: 7 (Mohs scale); Color: Gray, black, tan, or brown; Luster: Waxy to dull; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Gray, black, tan, or brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, black, tan, or brown; Luster: Waxy to dull; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from hydrothermal fluids in sedimentary environments. Common in limestone and chalk beds.
Uses & applications
Historically used for stone tools (arrowheads, knives) due to sharp edges; used in construction as road metal; sometimes polished as decorative stones or used as fire-starters (flint stones).
Geological facts
Chert was one of the most important materials in human prehistory for creating sharp-edged tools. The 'flintlock' mechanism in early firearms relied on flint's ability to create sparks when struck against steel.
Field identification & locations
Identify by its extremely smooth, shell-like (conchoidal) fracture and its ability to scratch glass. It is commonly found as nodules/concretions in limestone outcrops or as rounded river pebbles.
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