sedimentary
Sandstone Concretion
Arenaceous concretion (primarily SiO2 with Fe2O3 staining)
Hardness: 6-7 (Mohs); Color: Reddish-brown, terracotta; Luster: Dull/Earthy; Structure: Clastic, botryoidal-like tuberous form; Cleavage: None
- Hardness
- 6-7 (Mohs)
- Color
- Reddish-brown, terracotta
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (Mohs); Color: Reddish-brown, terracotta; Luster: Dull/Earthy; Structure: Clastic, botryoidal-like tuberous form; Cleavage: None
Formation & geological history
Formed by the precipitation of mineral cement (likely iron oxide or silica) within the spaces between sediment grains, often around a nucleus. They typically form in sedimentary basins during diagenesis.
Uses & applications
Used primarily as decorative garden stones, architectural accents, or as study specimens for geological education. Redder varieties are sometimes ground for natural pigments.
Geological facts
Concretions are often mistaken for fossils or meteorites due to their unusual, organic-looking shapes. Some can grow to several meters in diameter, such as the Moeraki Boulders in New Zealand.
Field identification & locations
Identify by its bulbous, 'tuber' shape and gritty texture. It will be harder than the surrounding host rock. Commonly found in eroded sandstone bluffs or riverbeds in arid regions like the American Southwest.
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