sedimentary
Ironstone Matrix with Jasper / Chert
Ferruginous Chert / Limonitic Jasper (SiO2 with Fe-oxides)
Mohs hardness of 6.0 to 7.0 (harder matrix and chalcedony parts, softer limonitic crusts around 4-5.5). Color ranges from reddish-brown, ochre yellow, to dark earthy brown with grayish quartzose patches. Dull to earthy luster on the weathered crust, sub-vitreous to waxy on fresher fracture surfaces.…
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Physical properties
Mohs hardness of 6.0 to 7.0 (harder matrix and chalcedony parts, softer limonitic crusts around 4-5.5). Color ranges from reddish-brown, ochre yellow, to dark earthy brown with grayish quartzose patches. Dull to earthy luster on the weathered crust, sub-vitreous to waxy on fresher fracture surfaces. Amorphous to microcrystalline mass with porous, irregular texture.
Formation & geological history
Formed in sedimentary weathering zones or secondary hydrothermal deposits where silica-rich fluids (chalcedony/quartz) interact with iron oxides like goethite, hematite, or limonite. Common in weathered basalt, laterite soils, and banded iron formations.
Uses & applications
Used primarily as a decorative rock specimen, lapidary material if containing cuttable agate/jasper pockets, and occasionally historically as a low-grade ore of iron.
Geological facts
Iron-stained jaspers and cherts often form inside nodules, geodes, or volcanic vugs where iron minerals oxidize in the presence of silica, producing distinctive rusty-red and yellow clay-like coats around denser quartz interiors.
Field identification & locations
Look for the heavy reddish-brown iron crust coating a hard microcrystalline silica mass. Scratching with a steel blade usually leaves a metal mark on the silica parts while scratching off the softer iron oxide coating. Commonly found in stream beds, desert pavements, and iron-rich mineralized districts worldwide.
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