Chert (River Pebble)

Physical Properties

Hardness: 6.5-7 (Mohs scale); Color: Gray, tan, brown, and yellow-orange iron staining; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic grains); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65.

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation from groundwater within sedimentary beds. This specimen is a water-worn river pebble, showing significant rounding from transport in water.

Uses & Applications

Historically used for stone tools (arrowheads/scrapers) due to its sharp conchoidal fracture. Modernly used for road aggregate, landscaping, and as a tumbling media for rock collectors.

Geological Facts

Chert is so durable that it often remains in riverbeds long after the surrounding limestone or shale has weathered away. Varieties of chert include flint, jasper, and agate.

Field Identification & Locations

Identify in the field by its smooth, waxy feel, extreme hardness (it will scratch glass/steel), and characteristic 'shell-like' conchoidal chips on the edges. Commonly found in riverbeds, beaches, and limestone outcrops.

Identified on: 4/26/2026

Mode: Standard