sedimentary

Ironstone Concretion (Iron-rich Mudstone)

Siderite / Geothite concretion (FeCO3 / FeO(OH))

Hardness: 3.5-5.5 (Mohs), Color: Dark brown to black with reddish oxidized patches, Luster: Submetallic to dull/earthy, Crystal structure: Amorphous or microcrystalline, Cleavage: None (conchoidal fracture when broken), Specific gravity: 3.0-4.0

Hardness: 3

Identified May 2, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of iron minerals within sedimentary layers, typically around a nucleus of organic material or mineral grain. Common in shale or sandstone beds, often dating from the Carboniferous to the Cretaceous periods.

Uses & applications

Historically used as a low-grade iron ore. Today, used as decorative stones, in landscaping, or as geological curiosities for collectors. Small, naturally shaped pieces are sometimes used in jewelry or as 'pocket stones'.

Geological facts

These are often mistaken for meteorites or fossils due to their unusual shapes and high density. Some concretions, known as 'Septarian nodules', feature internal cracks filled with other minerals like calcite.

Field identification & locations

Look for heavy, dark, often rounded or lumpy nodules in stream beds or eroding sedimentary cliff faces. They are significantly heavier than surrounding rocks and may show a 'rusty' outer layer where iron has oxidized.

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