Chert (Iron-Stained)

Physical Properties

Hardness: 7 (Mohs scale); Color: Rusty orange to reddish-brown (iron-stained); 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 chemical precipitation from groundwater. This specimen shows significant oxidation/iron-staining from surface weathering.

Uses & Applications

Historically used for stone tools (knapping) due to its sharp edges. Modernly used in construction aggregate, road base, and as decorative landscaping stone.

Geological Facts

Chert is so hard it can scratch steel and is incredibly resistant to chemical weathering. When struck with steel, it can produce sparks, leading to its historical use in flintlock firearms.

Field Identification & Locations

Identify in the field by its very smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. It often feels denser and smoother than surrounding sandstone or siltstone.

Identified on: 4/19/2026

Mode: Standard