sedimentary

Chert (Jasper)

Silicon dioxide (SiO2)

Hardness: 6.5-7 (Mohs); Color: Dark brown/black with reddish-brown patches; Luster: Waxy to dull (glassy when wet); Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific gravity: 2.6

Hardness: 6
Color: Dark brown/black with reddish-brown patches
Luster: Waxy to dull (glassy when wet)

Identified April 28, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the chemical precipitation of silica-rich fluids in sedimentary environments or as a replacement of organic material. These specimens are often reworked by water movement into rounded pebbles.

Uses & applications

Historically used for stone tools and weapons (flint knapping); currently used as river rock for landscaping, in aggregate, or as semi-precious gemstones for jewelry (cabs).

Geological facts

Chert and its variety Jasper were critical to human survival for thousands of years because they break with a conchoidal fracture, creating sharp edges for knives and arrowheads.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and smooth, shell-like fracture marks. Found globally in riverbeds, beaches, and sedimentary layers.