sedimentary

Geode / Hollow Concretion

Sedimentary Geode / Chalcedony-lined Concretion (primarily SiO2, CaCO3)

AI-generated identification · may be incorrect

Hardness: 6.5 - 7 on Mohs scale (for the siliceous rim), 3 - 4 (if calcareous); Color: Grayish-tan exterior with dark gray to earthy brown cavity fill; Luster: Dull, earthy to slightly vitreous; Structure: Sub-spherical hollow nodule, microcrystalline/cryptocrystalline to earthy; Fracture: Uneven to conchoidal.

Hardness: 6
Luster: Dull, earthy to slightly vitreous

Identified October 17, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in sedimentary rocks (such as limestone, shale, or mudstone) or volcanic layers. A cavity created by gas bubbles, dissolved fossils, or contracting mud fills inward with silica- or carbonate-rich groundwater over millions of years, often leaving a hollow or partially hollow center.

Uses & applications

Primarily collected as geological curiosities, ornamental pieces, and educational display specimens.

Geological facts

Geodes and hollow concretions can contain miniature crystal worlds inside, such as quartz, calcite, or chalcedony, though some remain lined only with fine-grained mud or microcrystalline chalcedony layers.

Field identification & locations

Recognized in the field by their rough, rounded, cauliflower-like exterior that feels lighter than a solid rock of the same size. Commonly found in stream beds, sedimentary limestone/shale beds, and volcanic ash deposits across the Midwestern US (e.g., Keokuk region), Mexico, and Brazil.