sedimentary

Polished banded chert

Chert (microcrystalline quartz, SiO₂), likely banded jasper/chert

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; dark gray to black with thin pale and rusty-brown bands, waxy to dull luster, very fine-grained cryptocrystalline structure, and conchoidal fracture. The rounded shape is polished or tumbled.

Identified August 30, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when silica-rich fluids replace or precipitate within sediment, commonly from microscopic marine organisms; individual layers may be Precambrian to much younger. Brown bands likely contain iron oxides or other impurities.

Uses & applications

Used for ornamental stones, cabochons, carvings, and decorative objects; historically, chert was an important material for tools and projectile points.

Geological facts

Chert can resemble flint, but “flint” commonly refers to dark chert associated with chalk. Its hardness allows it to scratch ordinary glass.

Field identification & locations

Identify by its glassy/waxy appearance, fine texture, lack of visible crystals, and conchoidal fracture; acid produces no fizz. Common in sedimentary formations worldwide, including the Great Lakes, Texas, England, and Australia.