Sedimentary or Igneous (secondary formation)
Quartz Geode (with Agate banding)
Silicon Dioxide (SiO2)
Hardness: 7 on Mohs scale; Color: Translucent white to clear (druzy quartz) with grey/white banding (chalcedony); Luster: Vitreous to waxy; Crystal structure: Trigonal/Hexagonal; Cleavage: None (conchoidal fracture).
- Hardness
- 7 on Mohs scale
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Translucent white to clear (druzy quartz) with grey/white banding (chalcedony); Luster: Vitreous to waxy; Crystal structure: Trigonal/Hexagonal; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed when mineral-rich groundwater seeps into hollow cavities (gas bubbles in volcanic rock or sedimentary animal burrows/tree roots). Over thousands of years, silica precipitates out of the water to form layers of agate and eventually inward-growing crystals.
Uses & applications
Predominantly used for ornamental purposes, jewelry (pendants/beads), mineral collecting, and interior decor.
Geological facts
Geodes are often referred to as 'thunder eggs' in certain regions. While they look like ordinary round rocks on the outside, their interior can contain various minerals including amethyst, calcite, or celestite.
Field identification & locations
Identify in the field by looking for rounded, hollow-sounding nodules that are lighter than surrounding rocks. Common in desert regions like Mexico, Brazil, and the Midwestern United States (Iowa/Illinois). Collectors should look for a botryoidal or bumpy exterior.
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Other sedimentary or igneous (secondary formation) specimens
Calcite
Calcium Carbonate (CaCO3)
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Quartz Geode (with possible Manganese or Goethite coating)
Silicon Dioxide (SiO2)
mineral
Hematite
Hematite (Iron(III) oxide, Fe2O3)
mineral
Hematite
Iron(III) oxide (Fe2O3)
mineral
Banded Agate
Chalcedony (Silicon Dioxide, SiO2)
mineral
Green Calcite
Calcium Carbonate (CaCO3)
mineral