sedimentary

Chert

Chert (SiO2)

Hardness: 7 on Mohs scale. Color: Brown, tan, reddish, and grayish hues due to iron oxides. Luster: Waxy to dull. Crystal Structure: Microcrystalline or cryptocrystalline quartz. Cleavage: None, exhibits conchoidal fracture. Specific Gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Brown, tan, reddish, and grayish hues due to iron oxides
Luster: Waxy to dull

Identified April 6, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by chemical precipitation in marine or groundwater environments. It typically forms as nodules or bedded deposits within limestone or shale.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges. Currently used in construction as road aggregate, in jewelry as semi-precious stones (jaspers), and as an abrasive.

Geological facts

Because it creates a spark when struck against steel, certain types of chert (flint) were essential for early firearms and fire-starting. It is one of the most durable sedimentary rocks and survives long after surrounding limestone has weathered away.

Field identification & locations

Identify by its extreme hardness (will scratch glass and steel), waxy texture, and smooth, curved (conchoidal) fractures. Common in river beds and weathered limestone outcrops worldwide.