sedimentary

Polished chert or jasper pebble

Microcrystalline quartz (SiO₂), likely chalcedony-rich chert/jasper

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; opaque brown, tan, and reddish coloration with a dull to waxy luster and fine, non-crystalline texture. It has no visible cleavage and typically breaks with conchoidal fracture; polishing has rounded and smoothed the surface.

Identified September 12, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater replaces or fills pores in sediment, commonly from biogenic silica or volcanic ash; iron oxides produce the red-brown colors. The specimen’s exact geological age cannot be determined from the image; chert occurs from Precambrian rocks to modern deposits.

Uses & applications

Used as ornamental stone, lapidary material, cabochons, beads, carvings, and occasionally as a source of prehistoric toolstone. Common polished pebbles generally have modest commercial value.

Geological facts

Jasper is an opaque, impure variety of chalcedony, while chert is usually named for its sedimentary origin and may be more uniformly fine-grained. Brown-red patterns commonly reflect hematite or goethite inclusions.

Field identification & locations

Identify it by its very fine texture, waxy appearance, lack of visible grains, and ability to scratch ordinary glass; a steel file generally leaves little mark. Common in gravel beds, river cobbles, sedimentary formations, and volcanic terrains worldwide.