sedimentary
Ironstone Concretion (Moqui Marble / Boji Stone)
Sedimentary Iron Oxide Concretion (Fe2O3 / FeO(OH))
Hardness: 5.0-6.0 Mohs. Color: reddish-brown, dark brown, to iron-gray. Luster: earthy to metallic. Crystal structure: amorphous or microcrystalline hematite/goethite matrix around a sandstone interior. Specific gravity: 3.0-4.5.
- Hardness
- 5
- Color
- reddish-brown, dark brown, to iron-gray
- Luster
- earthy to metallic
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Physical properties
Hardness: 5.0-6.0 Mohs. Color: reddish-brown, dark brown, to iron-gray. Luster: earthy to metallic. Crystal structure: amorphous or microcrystalline hematite/goethite matrix around a sandstone interior. Specific gravity: 3.0-4.5.
Formation & geological history
Formed in sedimentary rock formations, particularly sandstones, through the precipitation of iron mineral oxides dissolved in circulating groundwater around a organic nucleus or mineral core. Common in Mesozoic-aged sandstone beds.
Uses & applications
Used in novelty rock collecting, metaphysical practices, educational sets, and decorative specimens.
Geological facts
Ironstone concretions often form rounded, spherical, or irregular nodules. In Utah's Navajo Sandstone formation, smooth spherical iron concretions are locally known as 'Moqui Marbles'. Similar structures found on Mars by NASA's Opportunity rover were nicknamed 'blueberries'.
Field identification & locations
Identified by its distinctly heavy feel for its size, rounded/nodular form, magnetic attraction (sometimes weak), and dark reddish-brown earthy coating. Found in sedimentary basins worldwide, especially in arid sandstone regions like Utah and Arizona.
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