sedimentary

Ironstone Concretion

Ironstone (primarily Siderite or Limonite / Goethite, FeO(OH)·nH2O)

AI-generated identification · may be incorrect

Hardness: 5-5.5 on Mohs scale. Color: Deep reddish-brown to dark brown/ochre. Luster: Dull, earthy, or sub-metallic. Crystal Structure: Often amorphous or microcrystalline mass. Specific Gravity: 3.3-4.3.

Hardness: 5-5
Color: Deep reddish-brown to dark brown/ochre
Luster: Dull, earthy, or sub-metallic

Identified June 19, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of iron minerals within sedimentary layers, often around a nucleus like a leaf, shell, or fossil. These form in oxygen-poor environments like swamps or shallow marine basins over millions of years.

Uses & applications

Historically used as a low-grade iron ore. Today, it is primarily of interest to geological collectors and sometimes used as decorative garden stone or crushed for road base.

Geological facts

Ironstone concretions can sometimes contain perfectly preserved fossils inside if cracked open. They are often much harder than the surrounding sandstone or shale that they formed within.

Field identification & locations

Identify by its high weight (density) compared to other rocks and its characteristic rusty 'skin.' Commonly found in clay or sandstone beds. Collectors should look for rounded or nodular shapes that feel unusually heavy.