sedimentary

Ironstone Nodules / Siderite Concretion

Siderite (FeCO3) / Ferruginous Septarian Concretion

AI-generated identification · may be incorrect

Hardness: 3.5-4.5 (Mohs scale); Color: Dark brown, rusty orange, or dull grey; Luster: Earthy to sub-metallic; Crystal structure: Trigonal (often massive/concretionary); Specific Gravity: 3.8

Hardness: 3
Color: Dark brown, rusty orange, or dull grey
Luster: Earthy to sub-metallic

Identified March 14, 2026More sedimentary →

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Physical properties

Hardness: 3.5-4.5 (Mohs scale); Color: Dark brown, rusty orange, or dull grey; Luster: Earthy to sub-metallic; Crystal structure: Trigonal (often massive/concretionary); Specific Gravity: 3.8

Formation & geological history

Formed through the precipitation of iron minerals within sediment layers, often around a nucleus like a leaf, shell, or bone. These typically form in diagenetic environments within marine or swampy sedimentary basins across various geological ages.

Uses & applications

Historically used as a low-grade iron ore. Today, they are primarily of interest to amateur geologists and fossil hunters, as they often contain well-preserved fossils inside.

Geological facts

Many of the world's most famous fossils, such as those from the Mazon Creek formation, are found inside these types of iron-rich nodules. They are sometimes called 'cannonball stones' due to their rounded shapes.

Field identification & locations

Identify in the field by their heavy weight relative to size (high iron content) and distinct rounded or oblong shapes that stand out from surrounding shale or sandstone. Common in coal-bearing regions or coastal cliffs.