sedimentary
Chalcedony Geode Fragment
Silicon Dioxide (SiO2) - Microcrystalline Quartz
Hardness (Mohs): 6.5-7; Color: Translucent white to grey with green exterior (likely celadonite coating); Luster: Waxy to vitreous; Crystal structure: Trigonal/Monoclinic; Specific Gravity: 2.58-2.64
- Luster
- Waxy to vitreous
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Physical properties
Hardness (Mohs): 6.5-7; Color: Translucent white to grey with green exterior (likely celadonite coating); Luster: Waxy to vitreous; Crystal structure: Trigonal/Monoclinic; Specific Gravity: 2.58-2.64
Formation & geological history
Formed through the deposition of silica-rich groundwater within gas bubbles (vesicles) in volcanic rock or cavities in sedimentary rock. Over thousands of years, layers of chalcedony build up, sometimes leaving a hollow center.
Uses & applications
Primarily used for lapidary work, decorative display pieces, jewelry making, and as a popular specimen for educational or hobbyist collections.
Geological facts
The word geode is derived from the Greek word 'Geoides' meaning earth-like. Unlike a nodule which is solid, a geode must contain a hollow cavity, though this specimen is a broken fragment showing the inner chalcedony layer.
Field identification & locations
Look for rounded, lumpy rocks that feel lighter than they look. Field identification involves looking for chalcedony's distinctive waxy luster and conchoidal fracture. Common in areas with volcanic history such as Brazil, Uruguay, and the western US.
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