sedimentary
Ironstone Concretion (Moqui Marble)
Iron Oxide Concretion (Fe2O3 / FeO(OH) mixed with SiO2)
Hardness: 5-6 (outer shell); Color: Dark brown, rusty red, or black; Luster: Dull to earthy; Crystal structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 3.4-4.2
- Hardness
- 5-6 (outer shell)
- Color
- Dark brown, rusty red, or black
- Luster
- Dull to earthy
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Physical properties
Hardness: 5-6 (outer shell); Color: Dark brown, rusty red, or black; Luster: Dull to earthy; Crystal structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 3.4-4.2
Formation & geological history
Formed when iron-rich minerals precipitate from flowing groundwater within porous sedimentary rock (usually sandstone). They often form around a nucleus. They range from Jurassic to modern geological ages.
Uses & applications
Primarily used for geological study, educational specimens, and novelty collecting. Some spiritual practices use them as balancing stones.
Geological facts
Similar concretions have been discovered on Mars by the Opportunity rover, where they are nicknamed 'blueberries,' suggesting water once flowed on the Martian surface.
Field identification & locations
Identify by a hard, dark, iron-rich outer shell often surrounding a softer sandstone core. They are common in the Navajo Sandstone of Utah and Arizona. Look for spherical or 'pothole' shapes in desert washes.
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