mineral
Botryoidal chalcedony (likely agate/jasper variety)
Microcrystalline quartz, SiO₂
AI-generated identification · may be incorrectHardness 6.5–7; translucent to opaque, yellow, gray, blue-gray, and brown; waxy to dull luster; cryptocrystalline quartz with no visible cleavage and conchoidal fracture. The irregular, weathered piece appears naturally rough rather than polished.
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Physical properties
Hardness 6.5–7; translucent to opaque, yellow, gray, blue-gray, and brown; waxy to dull luster; cryptocrystalline quartz with no visible cleavage and conchoidal fracture. The irregular, weathered piece appears naturally rough rather than polished.
Formation & geological history
Forms when silica-rich groundwater fills cavities and fractures in volcanic rocks, often in successive colored layers or botryoidal masses. Usually Cenozoic to older volcanic-hosted material; exact age and locality cannot be determined from the image.
Uses & applications
Used for lapidary cabochons, beads, carvings, and ornamental stone; attractive specimens may be collected, while ordinary material has modest commercial value.
Geological facts
Yellow coloration may reflect iron oxides, while blue-gray areas can result from inclusions or chalcedony banding. Chalcedony is quartz whose crystals are too fine to see unaided.
Field identification & locations
Identify by its waxy appearance, translucent edges, conchoidal fracture, and hardness sufficient to scratch glass; it should not be confused with soft calcite or man-made slag. Common in basaltic volcanic districts worldwide, including Brazil, Mexico, Uruguay, India, and the western United States.
