sedimentary
Agate Geode
Silicon Dioxide (SiO2) - variety Chalcedony/Quartz
Hardness: 6.5-7 (Mohs scale); Color: White, brown, beige, translucent with banding; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline/cryptocrystalline chalcedony and macrocrystalline quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.60-2.65
- Hardness
- 6
- Color
- White, brown, beige, translucent with banding
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 (Mohs scale); Color: White, brown, beige, translucent with banding; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline/cryptocrystalline chalcedony and macrocrystalline quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.60-2.65
Formation & geological history
Formed as secondary deposits in cavities within host rocks, typically through the gradual deposition of silica from groundwater in volcanic vesicles or sedimentary limestone voids. These can be millions of years old, often originating in the Cenozoic or Mesozoic eras.
Uses & applications
Commonly used in lapidary arts for jewelry, decorative ornaments, bookends, and collection specimens. In industry, agate can be used for mortars and pestles due to its hardness and chemical resistance.
Geological facts
The word 'agate' comes from the Achates River in Sicily where they were first found. Geodes are unique because they are 'rock containers' that hide beautiful crystal structures inside that are only revealed when broken or cut.
Field identification & locations
Identify in the field by looking for rounded, lumpy nodules with a cauliflower-like exterior texture that feels heavier or lighter than expected for their size. This specimen shows classic banded agate transitions into a drusy quartz center. Common in Brazil, Uruguay, and the Midwestern United States.
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