sedimentary
Ironstone Concretion (Moqui Marble variety)
Ferruginous Sandstone Concretion
Hardness: 5-6 (outer shell); Color: Rusty brown, reddish, or tan; Luster: Dull or earthy; Structure: Spheroidal/Concentric layers of hematite or goethite around a sandstone core; Specific Gravity: 3.5-4.5.
- Hardness
- 5-6 (outer shell)
- Color
- Rusty brown, reddish, or tan
- Luster
- Dull or earthy
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Physical properties
Hardness: 5-6 (outer shell); Color: Rusty brown, reddish, or tan; Luster: Dull or earthy; Structure: Spheroidal/Concentric layers of hematite or goethite around a sandstone core; Specific Gravity: 3.5-4.5.
Formation & geological history
Formed through the precipitation of iron oxide minerals from groundwater within pore spaces of sedimentary rock (often sandstone). Generally formed during the Jurassic or Cenozoic periods.
Uses & applications
Primarily used for geological study, jewelry (beads), and as collector's specimens. Historically used by indigenous cultures as ceremonial stones.
Geological facts
In the United States, these are often called 'Moqui Marbles' and are famously found in the Navajo Sandstone of Utah. Similar structures have been observed on Mars, nicknamed 'Martian Blueberries'.
Field identification & locations
Identify by its distinct spherical or dome shape and heavy, iron-rich outer crust. They often weather out of softer sandstone cliffs. Common in desert environments with iron-rich groundwater history.
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