Chert (Beach Pebble)

Physical Properties

Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, white, and black swirls; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation in marine environments. This specimen has been rounded and polished by wave action on a beach.

Uses & Applications

Historically used for stone tools and starting fires (flint); currently used for decorative landscaping, jewelry (as river stones), and as a study specimen for sedimentary processes.

Geological Facts

Chert is extremely durable and chemically stable, which is why it often survives as smooth pebbles on beaches while softer rocks erode into sand. It is essentially the same material as flint, jasper, and agate.

Field Identification & Locations

Identify in the field by its hardness (it will scratch glass), its smooth waxy feel, and its curved conchoidal fractures. Commonly found in riverbeds and coastal shorelines where sedimentary layers have eroded.

Identified on: 5/3/2026

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