sedimentary
Chert (Smoky variety)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale. Color: Gray, brownish-gray to smoky black. Luster: Waxy to vitreous. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.65.
- Hardness
- 6
- Color
- Gray, brownish-gray to smoky black
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 on Mohs scale. Color: Gray, brownish-gray to smoky black. Luster: Waxy to vitreous. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through the chemical precipitation of silica in groundwater. Often found as nodules in limestone or chalk beds.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges. Currently used in construction aggregate, as a gemstone (when colorful, like jasper/agate), and for fire-starting in flint-and-steel kits.
Geological facts
Chert is so durable that it often survives as 'river rocks' long after the surrounding limestone has eroded away. The dark gray/black variety is specifically often referred to as 'Flint' in archeological contexts.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. It will not react with acid, unlike the limestone it is often found within. Common in the Midwest USA, UK, and limestone-heavy regions.
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