sedimentary
Geode (Quartz / Chalcedony Geode)
SiO2 (Silicon Dioxide)
Hardness: 6.5-7 on Mohs scale; Color: Tan/brown exterior with brownish-red/orange iron-stained interior; Luster: Earthy exterior, vitreous to dull interior; Crystal structure: Trigonal (microcrystalline/cryptocrystalline quartz); Cleavage: None.
- Hardness
- 6
- Luster
- Earthy exterior, vitreous to dull interior
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Tan/brown exterior with brownish-red/orange iron-stained interior; Luster: Earthy exterior, vitreous to dull interior; Crystal structure: Trigonal (microcrystalline/cryptocrystalline quartz); Cleavage: None.
Formation & geological history
Formed when mineral-rich water precipitates minerals within gas bubbles in volcanic rock or cavities in sedimentary rock layers over thousands to millions of years.
Uses & applications
Popular as decorative display specimens, educational samples, lapidary projects, and gemstone collecting.
Geological facts
Geodes appear to be ordinary, plain rocks on the outside, but cracking them open reveals hollow cavities lined with crystals. Iron oxide impurities often give the interior red, orange, or rusty yellow coloration.
Field identification & locations
Identified by a rounded, bumpy exterior shell combined with a lighter weight than solid rock of equivalent size due to the hollow interior cavity. Common in regions like the Midwestern US, Mexico, and Brazil.
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