sedimentary

Chert (Pebble)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Tan, brown, or buff; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz crystals); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 (Mohs scale)
Color: Tan, brown, or buff
Luster: Waxy to dull

Identified June 8, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or the chemical precipitation of silica from groundwater within sedimentary layers. These pebbles are typically worn smooth by water transport in rivers or beaches.

Uses & applications

Historically used for making stone tools (arrowheads/scrapers) due to its conchoidal fracture. Today used as construction aggregate, road base, and occasionally in landscape gardening or as a lapidary material for tumbling.

Geological facts

Chert is extremely durable and chemically stable, which is why it often survives as smooth pebbles in riverbeds long after surrounding rocks have eroded. In its dark gray to black form, it is commonly known as flint.

Field identification & locations

Identify in the field by its hardness (it will scratch glass), its smooth waxy feel, and its curved, shell-like (conchoidal) fracture pattern when broken. Commonly found in riverbeds, gravel pits, and coastal areas.