Chert (Iron-stained)

Physical Properties

Hardness: 6.5-7 on Mohs scale. Color: Brownish-tan due to iron oxide staining. Luster: Dull to waxy. 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 (like radiolaria or diatoms) or by chemical precipitation from groundwater within limestone or chalk beds. Often Paleozoic or Mesozoic in age.

Uses & Applications

Used historically for stone tools and arrowheads because of its conchoidal fracture. Today used as construction aggregate, road base, and occasionally as a flint for fire-starting.

Geological Facts

Chert is so hard it can scratch steel. When struck against steel, it produce sparks, which is why varieties like flint were essential for early survival. Iron staining gives this specimen its dull brown color.

Field Identification & Locations

Identify in the field by its hardness and scalloped (conchoidal) edges where it has broken. It will not react with acid, unlike the limestone it is often found in. Commonly found in stream beds or as nodules in carbonate rocks.

Identified on: 4/20/2026

Mode: Standard