sedimentary
Geode (Unopened)
Silica-lined Vug (primarily SiO2)
Hardness: 7 (exterior shell, typically chalcedony/chert); Color: Tan, white, or gray exterior; Luster: Dull/earthy exterior; Structure: Spherical to subspherical nodule; Specific Gravity: ~2.6-2.65
- Hardness
- 7 (exterior shell, typically chalcedony/chert)
- Color
- Tan, white, or gray exterior
- Luster
- Dull/earthy exterior
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Physical properties
Hardness: 7 (exterior shell, typically chalcedony/chert); Color: Tan, white, or gray exterior; Luster: Dull/earthy exterior; Structure: Spherical to subspherical nodule; Specific Gravity: ~2.6-2.65
Formation & geological history
Formed as hollow cavities in sedimentary rock (like limestone) or volcanic ash. Over millions of years, mineral-rich groundwater seeps into the cavity, depositing layers of crystals (usually quartz or calcite) on the interior walls. Age varies from 10 to 250+ million years depending on the host formation.
Uses & applications
Primarily used as decorative specimens for collectors, educational tools in geology, and occasionally cut into bookends or jewelry (druzy quartz).
Geological facts
Geodes are often described as 'geological surprise eggs' because their internal beauty is hidden until they are cracked open. The largest geode ever found is the 'Crystal Cave' in Ohio, measuring 35 feet in diameter.
Field identification & locations
Identify in the field by looking for unusually round, lumpy rocks that feel lighter than they look (if hollow). Common in desert regions or limestone beds. Collectors look for 'rattlers' where loose crystals can be heard inside.
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