sedimentary

Weathered gray-brown chert (flint)

Microcrystalline quartz, SiO₂ (chalcedony/chert)

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; opaque gray, brown, and rusty colors with dull to waxy luster, very fine-grained texture, and conchoidal fracture. No visible cleavage; density is typically about 2.6 g/cm³.

Identified August 16, 2026More sedimentary →

Explore Weathered gray-brown chert (flint) in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Forms when silica-rich groundwater replaces carbonate sediment or accumulates as siliceous sediment, commonly in limestone; most specimens are Paleozoic–Cenozoic, though chert occurs throughout geologic time. Brown staining likely reflects iron oxides and surface weathering.

Uses & applications

Used historically for sharp-edged tools and fire-starting, and occasionally as aggregate or decorative stone. Ordinary river-worn pieces have little commercial value.

Geological facts

The specimen’s rounded shape and smooth surface indicate transport in a stream or gravel deposit. Chert can produce a sharp edge and may spark against steel, although iron-bearing varieties often fracture less cleanly.

Field identification & locations

Identify by its dense, uniform texture, waxy appearance, lack of visible grains, and sharp conchoidal chips; a steel scratch test should generally fail. Common in limestone regions and stream gravels worldwide.