sedimentary
Quartz Geode
Silicon Dioxide (SiO2) within a geode
Hardness: 7 (Mohs scale), Color: White to translucent clear/gray, Luster: Vitreous to greasy, Crystal structure: Hexagonal (trigonal), Cleavage: None/Conchoidal fracture, Specific gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: White to translucent clear/gray, Luster: Vitreous to greasy, Crystal structure: Hexagonal (trigonal), Cleavage: None/Conchoidal fracture, Specific gravity: 2.65
Formation & geological history
Formed in sedimentary or volcanic rock cavities when mineral-rich water precipitates crystals inside hollow spaces over millions of years. Common in Mississippian and Devonian age strata (300-400 million years ago).
Uses & applications
Primarily used as decorative display specimens, for crystal healing collecting, and sometimes lower-grade material is used for jewelry (cabochons or drusy slabs).
Geological facts
Geodes are often indistinguishable from ordinary rocks on the outside. The Keokuk geode is the state rock of Iowa and is world-famous for its high concentration of quartz-filled geodes.
Field identification & locations
Identify in the field by looking for unusually light, round nodules that have a bumpy, 'cauliflower' exterior. They sound hollow when tapped. Commonly found in riverbeds and limestone outcrops.
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