sedimentary

Brown chert or flint pebble

Microcrystalline quartz (SiO₂), commonly with chalcedony and iron-oxide staining

AI-generated identification · may be incorrect

Typically Mohs hardness 6.5–7, opaque gray-brown to tan, waxy or dull luster, extremely fine-grained with no visible crystals or cleavage; conchoidal fracture and a dense feel. The specimen appears naturally water-rounded, with lighter vein-like zones and possible iron staining.

Identified August 18, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when silica replaces carbonate sediment or accumulates as biogenic silica in marine beds, later lithifying into nodules or layers. Chert ranges from Paleozoic to Cenozoic in age; this pebble was likely transported and rounded by a river, beach, or glacial process.

Uses & applications

Used historically for sharp tools and fire-starting, and today as a lapidary, landscaping, teaching, and decorative material. Ordinary pebbles have minimal commercial value.

Geological facts

Chert and flint are mineralogically similar; “flint” usually refers to dark, knappable chert. Its conchoidal fracture can produce razor-sharp edges.

Field identification & locations

Identify by testing for glass-like scratching ability, dense weight, lack of fizz in dilute hydrochloric acid, and conchoidal fracture; avoid destructive testing on valued pieces. Common worldwide, especially in limestone regions, river gravels, and glacial deposits.