sedimentary

Tumbled dark flint/chert

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

AI-generated identification · may be incorrect

Typically gray-black to brown, waxy to dull luster, opaque, very fine-grained cryptocrystalline structure, and conchoidal fracture; hardness about 6.5–7 Mohs, with no cleavage. The rounded edges indicate tumbling or natural abrasion, and reddish patches likely reflect iron oxides.

Identified August 16, 2026More sedimentary →

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Physical properties

Formation & geological history

Chert forms when silica-rich groundwater replaces or precipitates within limestone, marine sediments, or biogenic siliceous deposits, commonly during the Paleozoic–Cenozoic. It occurs as nodules, beds, or pebbles in sedimentary terrains.

Uses & applications

Used historically for sharp tools, arrowheads, and fire-starting; today it is mainly a lapidary, teaching, and collecting material, occasionally used as aggregate or decorative stone.

Geological facts

A steel file or glass-scratch test can help confirm its high hardness, while conchoidal chips distinguish it from many dull volcanic rocks. Common sources include the Great Lakes region, Texas, Kansas, England, and many gravel deposits.

Field identification & locations

Identify it by its smooth waxy appearance, lack of visible grains, strong resistance to scratching, and shell-like fracture; a fresh edge may be translucent. Avoid confusing it with obsidian, which is usually glassier and more uniformly vitreous.