sedimentary

Sandstone Concretion

Moqui Marble / Ironstone Concretion (Iron oxide-cemented sandstone)

AI-generated identification · may be incorrect

Hardness: 5-6 (Mohs scale), Color: Rusty brown, tan, or grey, Luster: Dull/Earthy, Crystal Structure: Amorphous/Granular matrix, Cleavage: None (breaks irregularly or along concentric layers), Specific Gravity: 2.5-3.0

Identified May 23, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of mineral cement in the spaces between particles in sediment. These typically form early in the burial history of the sediment, where iron or calcium carbonate concentrates around a nucleus like a leaf, shell, or fossil. Ages range from millions to hundreds of millions of years old.

Uses & applications

Used primarily as decorative collector specimens, educational pieces in geology, and sometimes carved for jewelry (specifically the Moqui Marble variety which is legally protected in certain areas).

Geological facts

Often mistaken for cannonballs or dinosaur eggs due to their spherical shape. The 'Moqui Marbles' found in Utah's Navajo Sandstone are some of the most famous examples and have been used by NASA scientists to study 'blueberries' (hematite concretions) found on Mars.

Field identification & locations

Identify by the spherical or disc-like shape that stands out from the surrounding rock layers. They are harder than the host rock, causing them to weather out and accumulate on the surface. Common locations include Utah, Kansas, and the United Kingdom.