sedimentary

Chert (River Pebble)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: light grey to tan with brownish veining; Luster: waxy to dull; Structure: microcrystalline/cryptocrystalline; Fractures: conchoidal; Specific gravity: approx 2.6

Hardness: 7 (Mohs scale)
Color: light grey to tan with brownish veining
Luster: waxy to dull

Identified July 6, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: light grey to tan with brownish veining; Luster: waxy to dull; Structure: microcrystalline/cryptocrystalline; Fractures: conchoidal; Specific gravity: approx 2.6

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or by chemical precipitation in saltwater or freshwater environments. The rounded shape suggests it was eroded and tumbled in a river or beach environment.

Uses & applications

Commonly used in ancient times for stone tools (knapping); today used in construction as road base or aggregate, and sometimes polished for decorative gardening stones.

Geological facts

Chert is extremely durable and chemically resistant. It often contains micro-fossils of marine organisms that lived millions of years ago. Flint is a well-known dark variety of chert.

Field identification & locations

Identifiable by its smooth, waxy feel, conchoidal fracture (curved breaks), and the fact that it can scratch glass. Look for them in dry creek beds or on coastal beaches among gravel deposits.