sedimentary

Ironstone with Pyrite

Ferruginous Sandstone (Fe2O3) with minor Sulfide minerals (FeS2)

AI-generated identification · may be incorrect

Hardness: 5-6.5 (Mohs); Color: Rusty brown, dark grey, with metallic brassy flecks; Luster: Earthy to metallic; Crystal structure: Isometric (pyrite inclusions) in an amorphous/granular matrix; Specific Gravity: 3.0-4.8 (variable due to iron content).

Hardness: 5-6
Color: Rusty brown, dark grey, with metallic brassy flecks
Luster: Earthy to metallic

Identified March 26, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the chemical precipitation of iron-rich sediments in maritime or swamp environments, often during the Proterozoic or Paleozoic eras. Oxidation of iron-rich minerals creates the brown 'rust' patina seen on the surface.

Uses & applications

Historical source of low-grade iron ore. Primarily used today as aggregate material in construction, as a pigment (ochre), or as landscape rock. Rare, well-crystallized specimens are collectors' items.

Geological facts

Ironstone nodules often contain fossils or minerals like pyrite (fool's gold). Some ironstones are so dense with magnetite that they can affect compass readings in the field.

Field identification & locations

Identify by its high density (heavy for its size), rusty appearance, and faint metallic glint. Commonly found in coal-bearing sedimentary basins or weathered coastal cliffs. Collectors should look for internal crystallization during cracking.