mineral
Tumbled chalcedony/agate (likely botryoidal variety)
Microcrystalline quartz, SiO₂; possibly druzy chalcedony or common agate
AI-generated identification · may be incorrectHardness about 6.5–7 Mohs; gray to black with a waxy-to-glassy luster and a rounded, polished surface. Quartz is trigonal but occurs here as cryptocrystalline material; visible pale drusy texture may indicate tiny quartz crystals.
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Physical properties
Hardness about 6.5–7 Mohs; gray to black with a waxy-to-glassy luster and a rounded, polished surface. Quartz is trigonal but occurs here as cryptocrystalline material; visible pale drusy texture may indicate tiny quartz crystals.
Formation & geological history
Forms when silica-rich groundwater fills cavities and vesicles in volcanic rock, precipitating chalcedony and quartz in layers or botryoidal crusts. Most such material is Cenozoic to younger, though the specimen’s age cannot be determined from the image.
Uses & applications
Used for tumbling, cabochons, carvings, beads, and ornamental objects; common pieces have modest jewelry and collector value.
Geological facts
The dark color may reflect manganese, iron, carbonaceous impurities, or artificial dye/coating. A glassy conchoidal chip and resistance to scratching by a steel knife support quartz identification.
Field identification & locations
Common sources include Brazil, Uruguay, Mexico, India, Madagascar, and the western United States. Look for curved banding, translucent edges, waxy luster, and hardness greater than glass; visual identification alone is not definitive.
