Chert (with Iron Staining)

Physical Properties

Hardness: 7 (Mohs scale), Color: Tan to grey with reddish-brown iron patina, Luster: Dull to waxy, Crystal structure: Microcrystalline (Cryptocrystalline), Cleavage: None (conchoidal fracture), Specific gravity: 2.65

Formation & Geological History

Formed through the accumulation of silica-rich microfossils (like radiolarians or diatoms) on the ocean floor or through chemical precipitation in groundwater. This specimen shows oxidation (rusting) from exposure to water and air.

Uses & Applications

Historically used for making sharp tools like arrowheads and scrapers. Currently used as road aggregate, in landscaping, and as a source of silica in glass manufacturing.

Geological Facts

Chert is so hard it can strike sparks against steel. It is chemically identical to quartz but has a much finer, denser internal structure. This specific piece is a river stone that has undergone significant chemical weathering.

Field Identification & Locations

Identify in the field by its very high hardness (it will scratch glass and steel) and its conchoidal (shell-like) fracture edges. Commonly found in river beds and sedimentary layers worldwide.

Identified on: 4/19/2026

Mode: Standard