sedimentary

Sandstone Chert Concretion

Silica-cemented Concretion (primarily SiO2)

AI-generated identification · may be incorrect

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

Identified March 14, 2026More sedimentary →

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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.