sedimentary

Chert (Pebble)

Microcrystalline silica (SiO2)

Hardness: 6.5-7 (Mohs scale); Color: Light blue-grey with tan staining; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 6
Color: Light blue-grey with tan staining
Luster: Waxy to dull

Identified May 5, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 (Mohs scale); Color: Light blue-grey with tan staining; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or chemical precipitation from groundwater within sedimentary beds. This specimen has been rounded by fluvial or lacustrine water action.

Uses & applications

Used historically for stone tools due to conchoidal fracture; modernly used in road construction, as a decorative landscape stone, and occasionally for lapidary tumbling.

Geological facts

Chert is one of the most durable sedimentary rocks. Varieties include flint and jasper. It was the primary material used by prehistoric humans to create arrowheads and scrapers.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and smooth, rounded water-worn texture. Commonly found in riverbeds, glacial till, and coastal shorelines.