sedimentary
Concretionary Sandstone
Clastic Sedimentary Rock (Sandstone with secondary mineral cementation)
Hardness: 6-7 (Mohs/quartz grains); Color: Tan, buff, or light brown; Luster: Dull or earthy; Crystal structure: Granular/Clastic; Cleavage: None; Specific Gravity: 2.2-2.8
- Hardness
- 6-7 (Mohs/quartz grains)
- Color
- Tan, buff, or light brown
- Luster
- Dull or earthy
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Physical properties
Hardness: 6-7 (Mohs/quartz grains); Color: Tan, buff, or light brown; Luster: Dull or earthy; Crystal structure: Granular/Clastic; Cleavage: None; Specific Gravity: 2.2-2.8
Formation & geological history
Formed by the precipitation of mineral cement (such as silica, calcium carbonate, or iron oxides) in the spaces between sediment grains. These often form around a nucleus in marine or terrestrial sedimentary basins, often dating from the Paleozoic to Mesozoic eras.
Uses & applications
Limited industrial use; primarily used as a landscape rock, architectural stone, or as a curiosity/collectible specimen due to its unique knobby shapes.
Geological facts
Concretions are often mistaken for fossils or dinosaur eggs because of their unusual, organic-looking shapes. The ‘Moqui Marbles’ of Utah are a famous small-scale example of iron-oxide concretions.
Field identification & locations
Identify by its bumpy, nodular texture which is harder than the surrounding host rock. Commonly found in eroded canyons, riverbeds, and sedimentary outcrops worldwide. Collectors look for symmetrical or highly unusual 'sculptural' forms.
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