sedimentary

Chert (River Pebble)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs); Color: Gray, bluish-gray; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (Microcrystalline quartz); Cleavage: None (Conchoidal fracture); Specific gravity: 2.65

Hardness: 7 (Mohs)
Color: Gray, bluish-gray
Luster: Waxy to dull

Identified June 25, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs); Color: Gray, bluish-gray; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (Microcrystalline quartz); Cleavage: None (Conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within limestone or chalk beds. This specimen has been rounded by fluvial (river) erosion.

Uses & applications

Historically used for making stone tools (arrowheads). Modern uses include road surfacing, decorative landscaping, and as a raw material for silicon production.

Geological facts

Chert is so hard it can strike sparks against steel. It is a very common constituent of river gravels because of its extreme resistance to chemical and physical weathering.

Field identification & locations

Identify by its 'flint-like' waxy texture, great hardness (it will scratch glass), and smooth, curved shell-like fractures. Found in riverbeds and coastal regions worldwide.