sedimentary or igneous (host environment)
Quartz Geode
Silicon Dioxide (SiO2) in a Geode body
Hardness: 7 (Mohs scale), Color: Clear/White crystals with tan/grey outer rind, Luster: Vitreous (internal) and earthy (external), Crystal Structure: Trigonal/Hexagonal, Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Clear/White crystals with tan/grey outer rind, Luster: Vitreous (internal) and earthy (external), Crystal Structure: Trigonal/Hexagonal, Specific Gravity: 2.65
Formation & geological history
Formed as hollow cavities in volcanic ash or sedimentary rock, later filled with mineral-rich water that precipitates crystals over millions of years. This specimen shows a transition from banded agate rinds to macro-crystalline quartz.
Uses & applications
Primarily used for decorative purposes, crystal collecting, metaphysical use, and teaching geology. Sometimes used in small-scale lapidary work.
Geological facts
Geodes are often called 'thunder eggs.' While they look like ordinary rocks on the outside, they contain hollow cavities that can house massive crystal clusters or even liquid trapped for millions of years.
Field identification & locations
Identify in the field by looking for rounded, lumpy nodules that feel lighter than they look (indicating a hollow center). Common locations include Brazil, Uruguay, and the Midwestern United States (Keokuk region).
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