sedimentary

Chert

Microcrystalline Quartz (SiO2)

Hardness: 6.5-7.0 (Mohs); Color: tan, grey, mottled brown; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.65

Hardness: 6
Color: tan, grey, mottled brown
Luster: waxy to dull

Identified June 18, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7.0 (Mohs); Color: tan, grey, mottled brown; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed in marine environments through the precipitation of silica, often as nodules in limestone or chalk through diagenesis. Can occur from the accumulation of silica-rich shells of plankton like radiolaria.

Uses & applications

Used historically for stone tools (lithics) like arrowheads and scrapers. Modern uses include road surfacing materials, grinding media, and decorative stones.

Geological facts

Because it breaks with a conchoidal fracture (like glass), it creates very sharp edges. It was one of the most vital resources for prehistoric humans. Some varieties of colorful chert are known as jasper.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by steel) and sharp, curved edges where it has fractured. Commonly found in stream beds or weathered out of carbonate rock outcrops. Look for a waxy texture.