sedimentary
Geode or Concretion
Silica-rich Concretion or Unopened Geode (SiO2)
Hardness: 7 (if quartz), Color: Tan to brown exterior, often with lichen; Luster: Dull/earthy exterior; Structure: Spherical or subspherical nodule; Specific Gravity: Approx 2.6.
- Color
- Tan to brown exterior, often with lichen
- Luster
- Dull/earthy exterior
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Physical properties
Hardness: 7 (if quartz), Color: Tan to brown exterior, often with lichen; Luster: Dull/earthy exterior; Structure: Spherical or subspherical nodule; Specific Gravity: Approx 2.6.
Formation & geological history
Formed through chemical precipitation around a nucleus in sedimentary rock layers, often limestone or shale, or via hydrothermal activity in volcanic cavities.
Uses & applications
Primarily used as collector specimens; if containing crystals, used for lapidary work and home decor.
Geological facts
Until crack opened, it is impossible to know if it is a solid concretion or a hollow geode filled with amethyst, quartz, or calcite crystals. Some ‘rattle stones’ have loose material inside.
Field identification & locations
Identify by its unusually round or oval shape compared to surrounding jagged rocks. Found in specific geological strata like the Keokuk area in the US or basalt beds in Brazil.
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