sedimentary

Ferruginous Sandstone Concretion (Ironstone Phosphorite)

Ferruginous Sandstone (variable Fe-oxide/hydroxide composition)

Hardness: 3-5 Mohs; Color: reddish-brown to dark chocolate brown; Luster: dull, earthy; Crystal structure: granular (clastic) with botryoidal or rippled surface; Cleavage: none; Specific gravity: 3.0-3.5.

Hardness: 3-5 Mohs
Color: reddish-brown to dark chocolate brown
Luster: dull, earthy

Identified June 27, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the precipitation of iron-oxide minerals like limonite or hematite between sediment grains. These often form in marine or fluvial environments during the Paleogene to Neogene periods.

Uses & applications

Primarily used as a collector specimen or decorative 'oddity'. Historically, ironstones were used as a low-grade iron ore in early industry.

Geological facts

Many concretions like this are mistaken for fossils (pseudo-fossils) due to their organic-looking rippled shapes, sometimes called 'Indian paint stones' or 'Moqui marbles' in different geological contexts.

Field identification & locations

Identify in the field by its high density compared to regular sandstone, its dark rusty streak when scratched, and its characteristic weathered, rounded, or rippled exterior. Common in the Southeastern United States and arid desert regions.

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