Chert (River Pebble)

Physical Properties

Hardness: 6.5-7 Mohs; Color: Brown, olive-green, or tan; Luster: Waxy to dull; Crystal structure: Microcrystalline/cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & Geological History

Formed through the accumulation of silica-rich microfossils (like radiolaria or diatoms) or by chemical precipitation in marine or groundwater environments. This specimen appears water-worn, indicating it was transported by a river or stream.

Uses & Applications

Historically used for making stone tools (arrowheads). Today used as decorative gravel, in road construction, and occasionally for lapidary work.

Geological Facts

Chert is so hard it can strike sparks against steel. Flint is a high-quality variety of chert. It is extremely resistant to weathering, which is why it often survives as smooth pebbles in riverbeds.

Field Identification & Locations

Identify in the field by its smooth, waxy texture and conchoidal (shell-like) fracture edges. Commonly found in riverbeds, gravel deposits, and limestone outcrops.

Identified on: 4/24/2026

Mode: Standard