sedimentary

Polished chert or jasper cobble

Microcrystalline quartz (SiO₂), commonly called chert; iron-oxide-bearing varieties are jasper

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; chiefly gray, brown, and reddish-orange with a waxy to dull luster and no visible cleavage. Its dense, cryptocrystalline quartz structure commonly produces smooth conchoidal fractures; the rounded form appears naturally water-worn and polished.

Identified August 10, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater replaces or precipitates within sediments, often during limestone or volcanic-sediment alteration; iron oxides create the rusty colors. Formation age depends on locality and may range from Paleozoic to Cenozoic.

Uses & applications

Used historically for tools and fire-starting, and today for lapidary carvings, cabochons, tumbled stones, and decorative objects; generally unsuitable as a major construction stone in this small form.

Geological facts

Jasper is an opaque, impurity-rich variety of chert, while cleaner gray areas are more typical chert or chalcedony. The specimen’s exact name requires a locality or testing because these varieties grade into one another.

Field identification & locations

Look for extreme hardness, fine grain, opacity, and conchoidal fracture; it should scratch ordinary glass but not be scratched by a steel knife. Common worldwide in sedimentary basins, river gravels, and hydrothermal zones.