sedimentary
Sandstone Concretion
Clastic Sedimentary Rock (primarily SiO2 with mineral cement)
Hardness: 6-7 (on Mohs scale for quartz grains); Color: Tan, beige, or brown with concentric banding; Luster: Dull/Earthy; Crystal structure: Granular/Clastic; Cleavage: None; Specific Gravity: 2.3–2.6.
- Hardness
- 6-7 (on Mohs scale for quartz grains)
- Color
- Tan, beige, or brown with concentric banding
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (on Mohs scale for quartz grains); Color: Tan, beige, or brown with concentric banding; Luster: Dull/Earthy; Crystal structure: Granular/Clastic; Cleavage: None; Specific Gravity: 2.3–2.6.
Formation & geological history
Formed by the precipitation of mineral cement within the spaces between sediment grains (usually sand). This often occurs around a nucleus, such as a leaf, shell, or bone, during diagenesis in sedimentary basins. They can range from thousands to millions of years old.
Uses & applications
Primarily used as decorative display pieces, garden stones, or educational geological specimens. In larger scales, sandstone is used in construction, but concretions are usually kept as curiosities.
Geological facts
Concretions are often mistaken for fossilized eggs or man-made artifacts because of their remarkably round or symmetrical shapes. Some famous examples include the Moeraki Boulders in New Zealand, which are massive versions of this phenomenon.
Field identification & locations
Identify in the field by looking for unusually round or disk-shaped rocks that differ in hardness or color from the surrounding host rock. They are commonly found in eroding shale or sandstone cliffs and along riverbeds. Collectors should look for nodules that have weathered out of their matrix.
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