mineral
Tumbled banded agate
Microcrystalline quartz (chalcedony), SiO₂, with band-forming impurities
AI-generated identification · may be incorrectHardness 6.5–7 Mohs; blue-gray, cream, and dark concentric bands; waxy to dull luster; cryptocrystalline quartz with no visible cleavage and conchoidal fracture. Specific gravity is about 2.6.
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Physical properties
Hardness 6.5–7 Mohs; blue-gray, cream, and dark concentric bands; waxy to dull luster; cryptocrystalline quartz with no visible cleavage and conchoidal fracture. Specific gravity is about 2.6.
Formation & geological history
Forms when silica-rich groundwater fills cavities in volcanic rock, depositing successive layers of chalcedony, often over millions of years. The specimen appears cut or naturally rounded and polished/tumbled.
Uses & applications
Used for cabochons, beads, ornaments, paperweights, and lapidary displays; durable and popular in inexpensive jewelry.
Geological facts
Agate banding records changing chemistry and fluid conditions during cavity filling. Individual patterns are highly variable, making each polished piece distinctive.
Field identification & locations
Identify by its fine concentric or irregular bands, translucency at thin edges, waxy surface, and lack of cleavage; common sources include Brazil, Uruguay, Mexico, India, and the United States. A hardness test should not scratch it with a steel knife.
