Ironstone Concretion (Moqui Marble)

Physical Properties

Hardness: 5-6 (Mohs); Color: Dark brown to reddish black exterior; Luster: Dull to sub-metallic; Crystal structure: Amorphous to microcrystalline; Cleavage: None; Specific Gravity: High (dense)

Formation & Geological History

Formed by the precipitation of iron oxide minerals around a nucleus (like a sand grain) within porous sandstone. They typically form from groundwater flow during the Jurassic or Cenozoic eras.

Uses & Applications

Primarily used as metaphysical 'worry stones,' teaching specimens in geology, and by collectors for their unique saucer-like or spherical shapes.

Geological Facts

Commonly known as Moqui Marbles (named after the Hopi tribe), similar ironstone concretions were discovered on Mars by the Opportunity rover and were nicknamed 'Martian blueberries.'

Field Identification & Locations

Identify by their distinct dark, heavy, metallic outer shell and sandy interior. They are often found weathering out of sandstone cliffs in the American Southwest, particularly Utah.

Identified on: 4/24/2026

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