sedimentary

Ironstone Concretion / Limonite

Goethite and Limonite (FeO(OH)·nH2O)

AI-generated identification · may be incorrect

Hardness: 4.0-5.5 (Mohs scale); Color: Rusty brown, yellowish-orange to dark blackish-red; Luster: Dull to earthy; Crystal structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 2.7-4.3.

Hardness: 4
Color: Rusty brown, yellowish-orange to dark blackish-red
Luster: Dull to earthy

Identified March 14, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of iron-oxide minerals within sedimentary layers, typically when iron-rich groundwater moves through porous sand or clay. These occur mostly in sedimentary basins and can range from any geological age from the Paleozoic to the present.

Uses & applications

Used historically as a low-grade iron ore. Today, they are primarily collected for geological interest, used in landscaping, or ground into pigments for 'ochre' paints.

Geological facts

These are often mistaken for meteorites because of their dark 'burnt' appearance and weight. They are also nicknamed 'Moqui Marbles' or 'shaman stones' in the American Southwest where they are particularly abundant.

Field identification & locations

Identify in the field by its heavy weight (high iron content), rusty streaks or crust, and a yellow-brown 'streak' test (scratching it on unglazed porcelain). Commonly found in eroded sandstone hillsides or desert pavement.