sedimentary
Ironstone Concretion (Moqui Marble Type)
Goethite/Hematite-cemented Sandstone Concretion
Hardness: 5-6 (outer shell); Color: Rusty brown, dark chocolate, or black; Luster: Dull or earthy; Crystal Structure: Amorphous or microcrystalline coating; Cleavage: None (conchoidal fracture when broken); Specific Gravity: 3.0-4.5 depending on iron content.
- Hardness
- 5-6 (outer shell)
- Color
- Rusty brown, dark chocolate, or black
- Luster
- Dull or earthy
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Physical properties
Hardness: 5-6 (outer shell); Color: Rusty brown, dark chocolate, or black; Luster: Dull or earthy; Crystal Structure: Amorphous or microcrystalline coating; Cleavage: None (conchoidal fracture when broken); Specific Gravity: 3.0-4.5 depending on iron content.
Formation & geological history
Formed by the precipitation of iron oxide minerals from groundwater flowing through permeable sedimentary rock (usually sandstone or shale). As the PH of the water changes, minerals collect around a nucleus such as a fossil or grain of sand, growing outwards. Often associated with Jurassic-aged formations (approx. 150-200 million years old).
Uses & applications
Used primarily as decorative stones, metaphysical 'pocket stones', and geological study specimens. Historically used by some Indigenous cultures as game pieces or sacred objects.
Geological facts
Identical structures have been found on Mars by the Opportunity rover and are nicknamed 'blueberries,' providing evidence that liquid water once flowed on the red planet. In the American Southwest, they are famously known as Moqui Marbles.
Field identification & locations
Identify in the field by their distinct spherical or disk-like shape and high weight compared to surrounding sandstone. They often weather out of cliffs and roll to the base. Commonly found in the Navajo Sandstone of Utah and Arizona, as well as the iron-rich clays of the UK and Southeast US.
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