sedimentary
Sandstone Chert Concretion
Silica-cemented Concretion (primarily SiO2)
Hardness: 6.5-7 (Mohs scale); Color: Tan, beige, or light brown; Luster: Dull or earthy; Crystal structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 6
- Color
- Tan, beige, or light brown
- Luster
- Dull or earthy
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Physical properties
Hardness: 6.5-7 (Mohs scale); Color: Tan, beige, or light brown; Luster: Dull or earthy; Crystal structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the precipitation of mineral cement (usually silica or calcite) between the grains of sediment. These typically grow outward from a nucleus within a sedimentary layer. Most range from Paleozoic to Cenozoic in age.
Uses & applications
Primarily used as a curiosity for collectors; larger varieties can be used in landscaping or as decorative garden stones. If sliced, interior patterns can be aesthetically interesting.
Geological facts
These are often mistaken for dinosaur eggs or fossils due to their organic, rounded shapes. They are known as 'Moqui Marbles' in specific regions of the US or 'Gogottes' in France, where they form exceptionally intricate shapes.
Field identification & locations
Identify in the field by their rounded or 'bloody' nodular shape which contrasts with the surrounding layered rock. Commonly found in weathered sandstone outcrops. Collectors should look for unusual symmetries or intact 'cluster' shapes.
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