sedimentary

Sandstone Concretion

Moqui Marble (Iron-oxide concretion)

Hardness: 5-5.5 (outer shell) to lower centered core; Color: Ochre, rusty brown, tan; Luster: Dull or earthy; Crystal structure: Amorphous or microcrystalline; Cleavage: None; Shape: Spherical to sub-spherical.

Hardness: 5-5
Color: Ochre, rusty brown, tan
Luster: Dull or earthy

Identified April 13, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the precipitation of mineral cement (usually hematite or goethite) within the spaces between sand grains. This process typically occurs in groundwater aquifers within sedimentary basins, often millions of years old.

Uses & applications

Used primarily as decorative curiosities, educational specimens in geology, and occasionally in metaphysical practices. Historically, native tribes used them as game stones.

Geological facts

In the American Southwest, these are often called 'Moqui Marbles' after the Hopi tribe. Similar spherical hematite concretions were discovered on Mars by the Opportunity rover, famously nicknamed 'blueberries'.

Field identification & locations

Identify by its distinct spherical shape and sandy texture. They are frequently found weathering out of sandstone formations, particularly the Navajo Sandstone in Utah and Arizona.

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