Banded Chert (Ironstone/Jasper)

Physical Properties

Hardness: 6.5-7 on Mohs scale. Color: Alternating bands of brown, tan, and dark grey/black. Luster: Waxy to vitreous. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.6 - 2.7.

Formation & Geological History

Formed through the accumulation of silica-rich organic remains or chemical precipitation in marine environments. The banding often represents seasonal or cyclical changes in deposition, frequently associated with Precambrian Banded Iron Formations (BIFs) dating back over 2 billion years.

Uses & Applications

Commonly used for lapidary work, decorative landscaping, and as a raw material for prehistoric stone tools due to its sharp conchoidal fracturing. Polished specimens are popular among rock collectors.

Geological Facts

This specimen appears to be a water-worn river cobble. Banded chert and jasper are among the oldest known rocks on Earth and provide evidence of the early atmosphere's oxygenation through the oxidation of dissolved iron.

Field Identification & Locations

Identify in the field by its high hardness (it will scratch glass), smooth surface when wet, and distinct parallel or contoured layering. Found frequently in gravel beds, glacial tills, and riverbasins in the Midwestern US and Canada.

Identified on: 4/21/2026

Mode: Standard