sedimentary
Chert (or Ironstone Nodule)
Silicon Dioxide (SiO2) or Limonite-stained Cryptocrystalline Quartz
Hardness: 6.5-7 (Mohs scale), Color: Brownish-tan to earthy yellow due to iron oxide staining, Luster: Dull to waxy, Texture: Dense and smooth-grained, Cleavage: None (conchoidal fracture).
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs scale), Color: Brownish-tan to earthy yellow due to iron oxide staining, Luster: Dull to waxy, Texture: Dense and smooth-grained, Cleavage: None (conchoidal fracture).
Formation & geological history
Formed through the accumulation of silica-rich fluids in sedimentary basins or as nodules within limestone/chalk, often enriched with iron minerals like limonite or goethite near the surface. Likely Cenozoic to Paleozoic in age depending on the local bedrock.
Uses & applications
Primarily used in construction as road aggregate or fill; historically used by indigenous peoples for stone tools due to its sharp breaking edges. Collected by amateurs as ornamental curiosities.
Geological facts
This specimen appears to be a stream-rounded or weathered nodule. Its earthy color is a result of chemical weathering where iron minerals have oxidized, essentially 'rusting' the surface of the silica-rich rock.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and lack of visible grains. It is often found in riverbeds, glacial till, or eroding out of limestone outcrops. Common across North America and Europe.
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