Chert (often called Flint or River Stone)

Physical Properties

Hardness: 7 on Mohs scale; Color: Tan, beige, and brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic); Cleavage: None (has conchoidal fracture); Specific gravity: 2.65

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in marine or freshwater environments. This specimen appears water-worn, likely from a river or glacial deposit.

Uses & Applications

Historically used for stone tools (arrowheads); currently used in road construction as aggregate, in landscaping, and as a durable lapidary material for tumbling.

Geological Facts

Chert is so hard that it can scratch steel. When struck against steel, it can create sparks, which is why darker varieties (flint) were used in flintlock firearms.

Field Identification & Locations

Identify by its waxy texture, extreme hardness, and 'shell-like' curved breaks (conchoidal fracture). It is extremely common in riverbeds and gravel pits worldwide.

Identified on: 4/15/2026

Mode: Standard