sedimentary

Chert (Smoky variety)

Microcrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

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

Identified March 25, 2026More sedimentary →

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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.