sedimentary

Tumbled chert or flint pebble

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

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; dark gray-brown with cream and rusty bands, waxy to dull luster, extremely fine-grained cryptocrystalline structure, and no visible cleavage. The smoothly rounded surface indicates natural water wear or tumbling.

Identified August 8, 2026More sedimentary →

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Physical properties

Formation & geological history

Chert forms when silica-rich groundwater replaces or fills microscopic pores in sediment, often in limestone or marine deposits; its age depends on the host formation, commonly Paleozoic to Cenozoic. Iron oxides create the brown and orange staining.

Uses & applications

Used historically for sharp tools and fire-starting, and today as decorative stone, lapidary material, landscaping aggregate, and a low-cost collectible pebble.

Geological facts

Freshly fractured chert commonly shows a curved, shell-like conchoidal fracture and sharp edges. Flint is usually a dark, compact variety of chert, while chalcedony-rich varieties may appear more translucent.

Field identification & locations

Identify it by its glassy or waxy appearance, very high hardness, lack of cleavage, and conchoidal fracture; it can scratch ordinary glass. Common in limestone regions, river gravels, and glacial deposits.