igneous
Geode, likely quartz-lined basalt geode
Amethyst/quartz geode in volcanic basalt; SiO₂ (quartz) with a basaltic host
AI-generated identification · may be incorrectDark gray, fine-grained outer host with a pale gray-white crystalline rim and small sparkling quartz crystals lining the cavity. Quartz has Mohs hardness 7, vitreous luster, hexagonal crystal structure, and no cleavage; basalt is typically ~6 Mohs and dense.
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Physical properties
Dark gray, fine-grained outer host with a pale gray-white crystalline rim and small sparkling quartz crystals lining the cavity. Quartz has Mohs hardness 7, vitreous luster, hexagonal crystal structure, and no cleavage; basalt is typically ~6 Mohs and dense.
Formation & geological history
Gas bubbles trapped in cooling basalt later became cavities, which were filled by silica-rich groundwater and crystallized quartz over millions of years, commonly during the Cenozoic. The specimen appears cut and polished across the cavity.
Uses & applications
Used as a decorative, educational, and mineral-collecting specimen; attractive amethyst varieties may be used in jewelry. Common quartz geodes generally have modest commercial value.
Geological facts
The outer dark material is likely basalt or another volcanic rock, while the pale band is crystalline quartz or chalcedony. The lack of strong purple color suggests ordinary quartz rather than amethyst.
Field identification & locations
Identify by a hollow or formerly hollow cavity, concentric chalcedony banding, sparkling quartz crystals, and a volcanic-looking exterior; common sources include Brazil, Uruguay, Mexico, Morocco, and the western United States. A loupe and hardness test help distinguish quartz from calcite.
