sedimentary

Chert (Jasper variety)

Silicon Dioxide (SiO2)

Hardness: 6.5 - 7 (Mohs scale); Color: Tan/cream with reddish-brown oxide bands; Luster: Dull to waxy; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6 - 2.7

Hardness: 6
Color: Tan/cream with reddish-brown oxide bands
Luster: Dull to waxy

Identified April 3, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich microfossils (like radiolaria or diatoms) or chemical precipitation from hydrothermal fluids in sedimentary basins. The reddish banding is typically caused by iron oxide impurities incorporated during formation.

Uses & applications

Historically used for stone tools (arrowheads/scrapers) due to conchoidal fracturing. Currently used for lapidary work, jewelry (cabs), and as decorative landscaping stones.

Geological facts

Chert was one of the most important minerals for early humans because its hardness and ability to hold a sharp edge made it ideal for toolmaking. This specific specimen shows signs of water-wearing, likely a river cobble.

Field identification & locations

Identify in the field by its hardness (it will scratch glass), waxy luster when broken, and characteristic curved (conchoidal) fractures. Often found in seafloor sedimentary deposits or as weathered pebbles in riverbeds.