Ironstone Concretion (Limonite/Goethite)

Physical Properties

Hardness: 4.0-5.5 on Mohs scale; Color: Dark brown to ochre yellow/orange; Luster: Dull, earthy, or sub-metallic; Crystal Structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 3.3-4.3

Formation & Geological History

Formed by the precipitation of iron-rich minerals in the pore spaces of sedimentary rocks or through the oxidation of iron-bearing minerals. They often form in wetlands, lake beds, or shallow marine environments as nodules over millions of years.

Uses & Applications

Historically used as a low-grade iron ore. Today, it is primarily used in landscaping, as a construction aggregate, or by amateur geologists for its unique aesthetic and internal banded structures.

Geological Facts

These are often mistaken for meteorites because of their high density and dark, rusty appearance. However, ironstones are much softer and lack the fusion crust found on meteorites. Many concretions contain a hollow center or a different colored core.

Field Identification & Locations

Identify by its high weight relative to its size (heft) and its yellow-orange streak when scratched on unglazed porcelain. Commonly found in eroded riverbeds, beaches, and areas with sedimentary clay or sandstone deposits.

Identified on: 4/22/2026

Mode: Standard