sedimentary

Ironstone Concretion / Jasper Pebble

Ferruginous chert / Siderite-Limonite concretion

AI-generated identification · may be incorrect

Hardness: 6-7 on the Mohs scale (for chert/jasper core) or 4-5.5 (if mainly iron oxide/hydroxide). Color: Ochre-brown, yellowish-brown with a dark metallic/blackish iron-manganese oxidation core or crust. Luster: Dull to earthy, sub-waxy where smooth. Fracture: Conchoidal to uneven.…

Hardness: 6-7 on the Mohs scale (for chert/jasper core) or 4-5
Luster: Dull to earthy, sub-waxy where smooth

Identified November 15, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by precipitation of iron minerals (such as limonite, goethite, or hematite) within sedimentary layers, or as water-worn pebbles of cryptocrystalline quartz (chert/jasper) heavily stained with iron oxides. It has been naturally smoothed by fluvial or marine tumbling.

Uses & applications

Primarily collected as curious landscape stones, decorative garden pebbles, or polishing rough. Historically, dense ironstone was occasionally used as a low-grade iron ore.

Geological facts

Weathered concretions and pebbles often develop differential staining called a 'desert varnish' or patinated rind due to long-term exposure to ground moisture, oxidation, and microbial deposition of manganese and iron oxides.

Field identification & locations

Identified in the field by its high relative weight, smooth rounded shape, yellow-to-dark brown iron-rich patina, and central dark patch where outer oxidized layers have weathered differently. Common in river gravels, glacial tills, and alluvial sediments worldwide.