sedimentary
Ironstone Concretion (Desert Varnish / Hematite-Goethite Nodule)
Iron oxide / hydroxide deposit (predominantly Fe2O3 and FeO(OH))
Hardness: 5.5 to 6.5 on the Mohs scale. Color: Dark brown to black with reddish-orange weathering patches. Luster: Submetallic to dull earthy. Cleavage: None (conchoidal to uneven fracture). Specific Gravity: High density (approx. 4.0-5.3 g/cm3), strongly heavy for its size.
- Hardness
- 5
- Color
- Dark brown to black with reddish-orange weathering patches
- Luster
- Submetallic to dull earthy
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Physical properties
Hardness: 5.5 to 6.5 on the Mohs scale. Color: Dark brown to black with reddish-orange weathering patches. Luster: Submetallic to dull earthy. Cleavage: None (conchoidal to uneven fracture). Specific Gravity: High density (approx. 4.0-5.3 g/cm3), strongly heavy for its size.
Formation & geological history
Formed by the precipitation of iron and manganese oxide minerals from groundwater within sedimentary strata, or by desert varnish coating weathering processes in arid environments over thousands to millions of years.
Uses & applications
Used primarily as iron ore when found in large deposits, in aggregate, for lapidary work, or collected as pseudo-meteorite specimens.
Geological facts
These heavy, dark, metallic-looking nodules are often mistaken for meteorites due to their fusion-crust-like surface (desert varnish) and high density, but they lack nickel and characteristic meteorite internal structures.
Field identification & locations
Field identification involves testing with a streak plate (hematite leaves a reddish-brown streak, goethite leaves a yellowish-brown streak) and testing magnetic attraction (usually weakly magnetic or non-magnetic compared to iron meteorites). Frequently found in arid or desert regions across North Africa, the Southwestern US, and Australia.
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