sedimentary

Chert (flint-like chalcedony)

Microcrystalline quartz, SiO₂

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; gray-beige, opaque to slightly translucent, with a dull to waxy luster and conchoidal fracture. It consists of cryptocrystalline quartz, commonly with iron-oxide staining and minor clay or carbonate impurities.

Identified August 8, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater replaces or cements sediment, often in limestone or chalk; its age depends on the host formation, commonly Paleozoic to Cenozoic. The rounded shape indicates transport and abrasion as a cobble, possibly with later surface weathering.

Uses & applications

Used historically for sharp tools and fire-starting, and today as a lapidary, carving, and decorative material; attractive patterned pieces may be polished. Ordinary specimens have little commercial value.

Geological facts

Chert and flint are compositionally similar; “flint” usually refers to dark, high-quality chert. Iron staining produces the tan and rusty patches visible here.

Field identification & locations

Identify by its hardness, lack of visible grains, waxy surface, and sharp conchoidal chips; it should scratch glass and resist a steel knife. Common in sedimentary terrains worldwide, especially limestone regions.