sedimentary
Sandstone Concretion
Clastic sedimentary rock (primarily Silica, SiO2, or Calcium Carbonate, CaCO3)
Hardness: 6-7 (if silica) or 3 (if calcite); Color: Tan, brown, or buff; Luster: Dull/Earthy; Crystal structure: Granular/Clastic; Cleavage: None; Specific Gravity: 2.2-2.8
- Hardness
- 6-7 (if silica) or 3 (if calcite)
- Color
- Tan, brown, or buff
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (if silica) or 3 (if calcite); Color: Tan, brown, or buff; Luster: Dull/Earthy; Crystal structure: Granular/Clastic; Cleavage: None; Specific Gravity: 2.2-2.8
Formation & geological history
Formed by the precipitation of mineral cement within the spaces between sediment grains (usually sand). Often grows outward from a nucleus like a fossil or leaf. Can take millions of years to form during diagenesis.
Uses & applications
Aesthetic display for collectors, garden ornaments, and occasionally studied by paleontologists as they may contain well-preserved fossils at the core.
Geological facts
Concretions are often mistaken for fossilized eggs or meteorites due to their rounded or unusual shapes. Some of the most famous are the Moeraki Boulders in New Zealand.
Field identification & locations
Identify by looking for a 'shell' and 'core' appearance. These are commonly found in eroding sedimentary bluffs or riverbeds. Collectors look for unbroken or perfectly spherical specimens.
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