mineral
Polished black agate (likely banded agate)
Microcrystalline quartz, SiO₂ (chalcedony)
AI-generated identification · may be incorrectSmall, tumbled, glossy dark pebble with a pale curved band; quartz hardness 6.5–7, trigonal crystal structure at microscopic scale, specific gravity about 2.6. Agate has no prominent cleavage and breaks with a conchoidal fracture.
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Physical properties
Small, tumbled, glossy dark pebble with a pale curved band; quartz hardness 6.5–7, trigonal crystal structure at microscopic scale, specific gravity about 2.6. Agate has no prominent cleavage and breaks with a conchoidal fracture.
Formation & geological history
Forms as silica-rich fluids fill cavities in volcanic rock, depositing successive layers of chalcedony; commonly dates from the host rock’s formation, often millions of years old.
Uses & applications
Used for jewelry, beads, carvings, and inexpensive collector specimens.
Geological facts
Agate’s characteristic bands form as successive silica layers, and polishing makes their patterns more visible.
Field identification & locations
Look for waxy-to-glassy luster, hardness, and curved or concentric bands; common sources include Brazil, Uruguay, India, and the United States. The image suggests a polished agate, but the exact variety cannot be confirmed from appearance alone.
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