mineral
Polished agate/geode fragment
Microcrystalline quartz (chalcedony), SiO₂, with possible calcite or quartz cavity lining
AI-generated identification · may be incorrectHardness about 6.5–7 Mohs; gray-beige, translucent chalcedony with a rough crystalline cavity and waxy-to-glassy luster. The rounded, polished exterior and hollow interior are characteristic of a small geode or agate nodule.
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Physical properties
Hardness about 6.5–7 Mohs; gray-beige, translucent chalcedony with a rough crystalline cavity and waxy-to-glassy luster. The rounded, polished exterior and hollow interior are characteristic of a small geode or agate nodule.
Formation & geological history
Forms when silica-rich groundwater fills cavities in volcanic rock, commonly basalt; layers crystallize inward over millions of years, typically during the host rock’s Cenozoic or older history.
Uses & applications
Used for cabochons, beads, ornaments, paperweights, and mineral collecting; attractive cavity crystals may modestly increase value.
Geological facts
Agate is a variety of banded or patterned chalcedony, while a geode is a hollow mineral-lined nodule; the specimen appears to combine both features.
Field identification & locations
Identify by its fine-grained, non-cleaving chalcedony rind, rounded nodule shape, and crystalline cavity; common sources include Brazil, Uruguay, Mexico, and the western United States.
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