sedimentary

Chert (Polished River Pebble)

Chert (SiO2)

Hardness: 7 (Mohs scale); Color: Cream, tan, and dark brown/black; Luster: Waxy to vitreous; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Hardness: 7 (Mohs scale)
Color: Cream, tan, and dark brown/black
Luster: Waxy to vitreous

Identified April 28, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Cream, tan, and dark brown/black; Luster: Waxy to vitreous; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within sedimentary layers. This specific specimen has been water-worn and polished in a river or coastal environment.

Uses & applications

Used for road surfacing, lapidary tumbling, and historically for stone tools (arrowheads). Small polished pieces are popular for decorative gardens and aquarium gravel.

Geological facts

Chert is chemically nearly identical to quartz but has a microcrystalline structure. Its tendency to break with sharp edges made it the primary material for prehistoric cutting tools.

Field identification & locations

Identify in the field by its hardness (scratches glass), waxy feel, and smooth surfaces when water-worn. Common in limestone formations and riverbeds worldwide.