sedimentary

Tumbled chert or flint pebble

Microcrystalline quartz (SiO₂), commonly chalcedony/chert

AI-generated identification · may be incorrect

Brown, gray, and cream mottling with a dull-to-waxy luster and rounded, polished surface; hardness about 6.5–7 Mohs, no visible cleavage, cryptocrystalline structure, and specific gravity near 2.6. Small pale inclusions may be quartz, calcite, or fossil fragments.

Identified September 3, 2026More sedimentary →

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Physical properties

Formation & geological history

Chert forms when silica-rich groundwater replaces or cements marine or freshwater sediments, often preserving microscopic fossils; the specimen was later rounded by stream or wave abrasion and likely tumbled. Individual deposits range from Precambrian to Cenozoic in age.

Uses & applications

Used historically for sharp-edged tools and fire-starting, and today as decorative gravel, lapidary material, and inexpensive polished stones.

Geological facts

Freshly fractured chert produces a conchoidal fracture and very sharp edges; brown varieties are often called jasper when strongly opaque and iron-stained.

Field identification & locations

Identify by its glassy/conchoidal fracture, resistance to scratching by steel, and inability to scratch quartz; common in limestone regions, river gravels, and glacial deposits. The image alone cannot distinguish chert from jasper with certainty.