sedimentary
Iron-Oxide Concretion (Moqui Marble)
Goethite/Hematite Concretion (Fe2O3 / FeO(OH))
Hardness: 5-5.5 (outer shell); Color: Brown, reddish-brown, or greyish-black; Luster: Submetallic to earthy; Structure: Spherical to subspherical; S.G.: ~3.4-5.0.
- Hardness
- 5-5
- Color
- Brown, reddish-brown, or greyish-black
- Luster
- Submetallic to earthy
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Physical properties
Hardness: 5-5.5 (outer shell); Color: Brown, reddish-brown, or greyish-black; Luster: Submetallic to earthy; Structure: Spherical to subspherical; S.G.: ~3.4-5.0.
Formation & geological history
Formed by the precipitation of iron-bearing minerals around a nucleus (often sand) within groundwater-saturated sedimentary rock like sandstone. Most famous specimens are from the Jurassic Navajo Sandstone (approx. 180-190 million years old).
Uses & applications
Primarily used for geological study, mineral collecting, and metaphysical purposes. Some larger iron concretions have historically been used as a source of iron ore.
Geological facts
In the American Southwest, these are known as 'Moqui Marbles' named after the Hopi tribe. Similar structures called 'Blueberries' were discovered on Mars by the Opportunity rover, indicating the historical presence of water.
Field identification & locations
Identify by their distinct spherical shape and heavy iron-rich outer shell. Frequently found in the desert Southwest (Utah, Arizona) where they weather out of eroding sandstone cliffs.
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