sedimentary

Weathered chert or flint nodule

Microcrystalline quartz (SiO₂), commonly with chalcedony and minor iron oxides

AI-generated identification · may be incorrect

Typically Mohs hardness 6.5–7, dull to waxy luster, gray/tan/brown coloration, and very fine-grained cryptocrystalline structure; it has no visible cleavage and breaks with curved, conchoidal fractures. The layered, weathered appearance may reflect included limestone or iron-stained material.

Identified August 13, 2026More sedimentary →

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Physical properties

Formation & geological history

Chert forms when silica replaces carbonate sediment or accumulates from microscopic marine organisms, generally during sedimentary burial and diagenesis. Its exact age cannot be determined from the photograph; nodules occur in sedimentary formations of many ages.

Uses & applications

Used historically for sharp tools, arrowheads, and fire-starting; crushed chert is used as aggregate and road material. Ordinary specimens have little jewelry value, though colorful polished pieces are collectible.

Geological facts

Flint is a dark, often impure variety of chert; freshly fractured surfaces are sharper than glass. Iron oxides commonly produce reddish or yellow-brown staining.

Field identification & locations

Look for a very hard, fine-grained surface that scratches glass and produces a sharp conchoidal chip; it should not fizz strongly in dilute acid unless coated with limestone. Common in limestone and chalk regions worldwide.