sedimentary

Chert

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: tan, brown, and cream with dark inclusions; Luster: waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: none, exhibits conchoidal fracture.

Hardness: 7 on Mohs scale
Color: tan, brown, and cream with dark inclusions
Luster: waxy to dull

Identified June 23, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: tan, brown, and cream with dark inclusions; Luster: waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: none, exhibits conchoidal fracture.

Formation & geological history

Formed primarily as nodules or layers in limestone or chalk through the accumulation of siliceous remains of marine organisms (like diatoms or radiolarians) or through chemical precipitation from silica-rich fluids.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges when fractured. Modernly used as road surfacing material or decorative gravel.

Geological facts

Because it is so hard and chemically stable, chert is often found as smooth pebbles in riverbeds or on beaches long after the surrounding softer rock has eroded away.

Field identification & locations

Identify in the field by its hardness (will scratch glass), waxy appearance, and smooth, curved (conchoidal) fracture surfaces. Commonly found in sedimentary basins and limestone outcrops.