Limonite Concretion

Physical Properties

Hardness: 4-5.5 (Mohs scale); Color: Tan, yellow-brown to earthy orange; Luster: Dull or earthy; Crystal structure: Amorphous; Cleavage: None; Specific gravity: 2.7-4.3

Formation & Geological History

Formed through the chemical precipitation of iron-rich minerals in sedimentary environments. These often form around a nucleus (like a shell or grain) as groundwater trickles through porous sand or clay, leading to localized iron oxide buildup.

Uses & Applications

Historically used as a low-grade iron ore and as a pigment (ochre). Collected by hobbyists for unique shapes and curious textures.

Geological Facts

Limonite is not a true mineral but a mixture of hydrated iron oxide minerals, mostly goethite. It has been used as a pigment for cave paintings dating back thousands of years.

Field Identification & Locations

Identify by its characteristic 'rusty' yellow-brown streak when rubbed against tile. Often found in sandstone beds, riverbanks, or areas with heavy weathering of iron-rich rocks. It feels relatively heavy for its size due to iron content.

Identified on: 4/17/2026

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