Ironstone Concretion (Limonite/Goethite)

Physical Properties

Hardness: 4-5.5 on Mohs scale; Color: Rusty brown, ochre, or dark tan; Luster: Dull to earthy; Crystal Structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 2.7-4.3.

Formation & Geological History

Formed by the precipitation of iron oxide minerals within the spaces between sediment grains, often around a nucleus like a leaf, shell, or fossil. These typically form in sedimentary basins or soil horizons where groundwater is rich in dissolved iron.

Uses & Applications

Historically used as a low-grade iron ore. Today, used primarily for decorative landscaping, educational specimens, or occasionally as a pigment (ochre).

Geological Facts

These are often mistaken for meteorites or 'fossilized poop' (coprolites) due to their strange, organic shapes. They can range from marble-sized 'Moqui marbles' to several feet in diameter.

Field Identification & Locations

Identify in the field by its heavy feel relative to its size, rusty streak when rubbed on unglazed porcelain, and lack of internal crystalline structure. Common in sandstone or shale formations.

Identified on: 4/19/2026

Mode: Standard