sedimentary
Sandstone Concretion
Clastic Sedimentary Concretion (SiO2 with Iron Oxide)
Hardness: 6-7 (Mohs); Color: Tan, brown, ochre; Luster: Dull/Earthy; Crystal structure: Amorphous/Granular clastic; Cleavage: None; Specific gravity: 2.3-2.6
- Hardness
- 6-7 (Mohs)
- Color
- Tan, brown, ochre
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (Mohs); Color: Tan, brown, ochre; Luster: Dull/Earthy; Crystal structure: Amorphous/Granular clastic; Cleavage: None; Specific gravity: 2.3-2.6
Formation & geological history
Formed by the precipitation of mineral cement (often calcite or iron oxide) within the spaces between sediment grains, usually around a nucleus like a shell or leaf. They form in sedimentary basins over millions of years.
Uses & applications
Used primarily as decorative garden stones, oddities for collectors, and occasionally used in landscaping. Historically, some were used as primitive tools or pigments.
Geological facts
Concretions are often mistaken for fossils, eggs, or man-made artifacts because of their symmetrical or unusual shapes. The Moqui Marbles are a famous type of sandstone concretion found in Utah.
Field identification & locations
Identified by its rounded or bulbous shape that differs from the surrounding rock layers. Often found in eroded sandstone cliffs or riverbeds. Look for concentric layering or 'capping' on the outer shell.
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