sedimentary
Ironstone Concretion / Goethite-Hematite Nodule
Goethite/Hematite (α-FeO(OH) / Fe2O3)
Hardness: 5.0–6.5; Color: Rusty brown, dark reddish-brown, or black; Luster: Dull, earthy to sub-metallic; Structure: Often concretionary or botryoidal; Gravity: 3.3–4.3
- Hardness
- 5
- Color
- Rusty brown, dark reddish-brown, or black
- Luster
- Dull, earthy to sub-metallic
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Physical properties
Hardness: 5.0–6.5; Color: Rusty brown, dark reddish-brown, or black; Luster: Dull, earthy to sub-metallic; Structure: Often concretionary or botryoidal; Gravity: 3.3–4.3
Formation & geological history
Formed through the precipitation of iron-rich minerals from groundwater within sedimentary rock layers, often as secondary mineralization. These can range from modern formations to hundreds of millions of years old.
Uses & applications
Primarily used as an ore of iron in large deposits. For individual collectors, they are kept as curiosities, lapidary material, or naturally occurring 'charms'. Redder Varieties provide natural ochre pigments.
Geological facts
Many naturally occurring ironstone nodules are mistaken for meteorites because of their high density and dark, weathered appearance. They are sometimes called 'blueberries' when found in small, spherical forms on Mars.
Field identification & locations
Identify by its high density relative to other rocks, a reddish-brown streak test on unglazed porcelain, and its presence in gravel beds or creek bottoms. Often found in proximity to sandstone and shale.
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