sedimentary

Chert (River Pebble)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Brown, tan, olive-green; Luster: Waxy to vitreous when wet; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 (Mohs scale)
Color: Brown, tan, olive-green
Luster: Waxy to vitreous when wet

Identified July 4, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Brown, tan, olive-green; Luster: Waxy to vitreous when wet; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Formation & geological history

Formed through the accumulation of silica-rich microfossils (like radiolaria or diatoms) or chemical precipitation from groundwater within sedimentary beds. This specimen has been rounded by fluvial chemical and physical weathering in a river environment.

Uses & applications

Used historically for stone tools due to conchoidal fracturing; modern uses include construction aggregate, road surfacing, and as a landscaping stone.

Geological facts

Chert is extremely durable and chemically stable, which is why it often remains as pebbles long after the surrounding host rock has eroded away. It was one of the most important materials for prehistoric humans to create sharp edges.

Field identification & locations

Identify by its hardness (it will scratch glass), its smooth waxy feel, and the lack of visible grains. Common in riverbeds, gravel pits, and limestone outcrops. Collectors look for unique colors or banding.