sedimentary
Quartz Geode
Silicon Dioxide (SiO2) within a secondary sedimentary structure
Hardness: 7 (Mohs scale); Color: Clear to white internal crystals with a brownish-gray outer shell; Luster: Vitreous (glassy) crystals; Crystal Structure: Hexagonal/Trigonal; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Luster
- Vitreous (glassy) crystals
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Physical properties
Hardness: 7 (Mohs scale); Color: Clear to white internal crystals with a brownish-gray outer shell; Luster: Vitreous (glassy) crystals; Crystal Structure: Hexagonal/Trigonal; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through the precipitation of minerals from hydrothermal fluids or groundwater within cavities of sedimentary rocks (like limestone) or volcanic ash. These cavities are often created by gas bubbles or decomposed organic matter. They can date from millions to hundreds of millions of years old.
Uses & applications
Primarily used for geological education, home decor, and mineral collecting. Large-scale quartz is used in industry for electronics (piezoelectric properties) and glassmaking, though geodes are mainly aesthetic specimens.
Geological facts
Geodes are often referred to as 'thunder eggs' in certain regions. While they look like ordinary round rocks on the outside, they are hollow and lined with crystals. The largest geode ever found is the 'Empress of Uruguay,' which stands over 11 feet tall.
Field identification & locations
Identify in the field by looking for rounded, nodular rocks that feel lighter than they look (indicating a hollow center). Common locations include the Keokuk region (Iowa/Illinois/Missouri), Morocco, Brazil, and Mexico. For collectors, look for intact shells with well-defined, sparkling crystals.
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