sedimentary

Chert (specifically Flint or Dark Chalcedony)

Silicon Dioxide (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Black, dark brown, or grey with translucent edges; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline (hexagonal system at microscopic level); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 6
Color: Black, dark brown, or grey with translucent edges
Luster: Waxy to vitreous

Identified May 19, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of siliceous remains of organisms (like diatoms or radiolaria) or by chemical precipitation from silica-rich fluids within sedimentary rock layers, often limestone or chalk. Many specimens date from the Cretaceous to the Jurassic periods.

Uses & applications

Historically used for stone tools (knapping) and fire-starting. Today used in jewelry as cabochons, as road aggregate, and for decorative specimen collecting.

Geological facts

Chert is so hard it can scratch steel. The term 'flint' is often reserved for chert found in chalk or marly limestone. It was the primary material for the Stone Age technology that allowed early humans to hunt and survive.

Field identification & locations

Identify by its smooth, shell-like (conchoidal) fracture pattern and its ability to spark when struck against steel. Found globally in sedimentary basins, riverbeds, and glacial tills.

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