Chert (Polished Gemstone / River Stone)

Physical Properties

Hardness: 6.5-7 on Mohs scale; Color: Grayish-tan or olive-gray; Luster: Waxy to vitreous (polished); Crystal Structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & Geological History

Formed through the accumulation of silica-rich marine organisms (like diatoms or radiolarians) or the chemical precipitation of silica in groundwater. This specimen appears water-worn or mechanically tumbled.

Uses & Applications

Commonly used as decorative landscape stones, tumbled pocket stones, or historically for tool-making (flintknapping) due to its sharp edges when broken.

Geological Facts

Chert is a high-silica rock that includes varieties like flint, jasper, and agate. It is incredibly durable and resistant to chemical weathering, which is why it often survives as smooth pebbles in riverbeds.

Field Identification & Locations

Identify in the field by its hardness (cannot be scratched by a steel knife) and its smooth, waxy feel. It often has a conchoidal (shell-like) fracture pattern if broken.

Identified on: 4/15/2026

Mode: Standard