sedimentary
Sandstone Concretion
Clastic Sedimentary Concretion (primarily SiO2 with Fe2O3 cement)
Hardness: 6-7 (Mohs); Color: Brown, tan, or rust-colored; Luster: Dull/Earthy; Crystal structure: Microcrystalline/Granular; Cleavage: None; Specific Gravity: 2.5-2.8.
- Hardness
- 6-7 (Mohs)
- Color
- Brown, tan, or rust-colored
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (Mohs); Color: Brown, tan, or rust-colored; Luster: Dull/Earthy; Crystal structure: Microcrystalline/Granular; Cleavage: None; Specific Gravity: 2.5-2.8.
Formation & geological history
Formed by the precipitation of mineral cement (like iron oxide or calcite) within the spaces between sediment grains, often around a nucleus like a leaf or shell. This specimen appears to be an iron-rich (ferruginous) sandstone concretion.
Uses & applications
Primarily of value to collectors, educators, and geologists. Some types are used in landscaping, while larger formations are tourist attractions (e.g., Moeraki Boulders).
Geological facts
Concretions are often mistaken for fossils, eggs, or meteorites due to their unusual globular shapes. The 'Moki Marbles' found in Utah are famous examples of this geological phenomenon.
Field identification & locations
Identify in the field by looking for rounded, nodular shapes that differ in hardness and color from the surrounding host rock. Commonly found in areas with exposed sedimentary layers like riverbeds or deserts.
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