sedimentary
Chert (Flint variety)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, brown, or dull black; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Gray, brown, or dull black
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, brown, or dull black; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic debris (like sponge spicules or radiolarians) or by chemical precipitation from hydrothermal fluids in limestone or chalk beds. Most common in Paleozoic and Mesozoic strata.
Uses & applications
Historically used for stone tools and weapons (arrowheads). Modernly used in construction aggregate, road surfacing, and as a component in glass and ceramics. High-quality pieces are used for lapidary work.
Geological facts
Flint was one of the most important resources for prehistoric humans because of its ability to produce sharp edges and sparks when struck against steel or pyrite. It is often found as nodules inside chalk or limestone deposits.
Field identification & locations
Identify in the field by its conchoidal (shell-like) fracture pattern and its ability to scratch glass. It often has a white, chalky outer 'rind' or cortex. Found globally, especially in regions with deep limestone beds such as the UK, France, and the Midwestern United States.
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