sedimentary
Quartz Geode (Unopened)
Silicon Dioxide (SiO2) with Iron Oxides
Hardness 7 on the Mohs scale (for quartz contents), rough, pitted, and bumpy exterior rind, yellowish-tan to rusty brown iron-stained coloration, dull to earthy luster on the outside, light in weight relative to its size if hollow.
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Physical properties
Hardness 7 on the Mohs scale (for quartz contents), rough, pitted, and bumpy exterior rind, yellowish-tan to rusty brown iron-stained coloration, dull to earthy luster on the outside, light in weight relative to its size if hollow.
Formation & geological history
Formed in sedimentary rocks like limestone and shale through precipitation of silica from groundwater within underground voids, gas bubbles, or organism burrows. Commonly dated from Mississippian to Cretaceous eras.
Uses & applications
Popular among rockhounding hobbyists, mineral collectors, and lapidary artists for cracking open to reveal sparkling quartz, amethyst, or chalcedony crystals inside.
Geological facts
The rough, bumpy, cauliflower-like surface with iron staining is a classic sign of a geode nodule. Collectors often crack or saw them open to discover hidden crystal cavities inside.
Field identification & locations
Identified in the field by its rounded nodular shape, dimpled 'cauliflower' surface texture, and lighter-than-expected weight if hollow inside. Commonly found in stream beds and limestone outcrops in states like Iowa, Illinois, and Missouri (e.g., Keokuk region).
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