sedimentary
Geode with Quartz and Chalcedony
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale); Color: Gray, brown, and translucent white crystals; Luster: Vitreous to waxy; Crystal Structure: Trigonal/Hexagonal; Cleavage: None (Conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Gray, brown, and translucent white crystals
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, brown, and translucent white crystals; Luster: Vitreous to waxy; Crystal Structure: Trigonal/Hexagonal; Cleavage: None (Conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through the precipitation of mineral-rich groundwater within hollow cavities (vugs) in sedimentary rock layers. Slowly cooling silica-rich fluids deposit layers of chalcedony and eventually crystalline quartz over millions of years.
Uses & applications
Primarily used for geological education, home decor, and mineral collecting. Polished specimens may be used in jewelry or as paperweights.
Geological facts
Geodes are often referred to as 'thunder eggs' in certain regions. While the outside looks like an ordinary rock, the interior can contain a variety of minerals including amethyst, calcite, or celestite.
Field identification & locations
Identify in the field by looking for rounded, bumpy, or lumpy nodules that feel unusually light for their size. Commonly found in limestone deserts or volcanic ash beds. Best found in regions like the American Southwest or Brazil.
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