mineral
Tumbled banded agate
Chalcedony (microcrystalline quartz), SiO₂
AI-generated identification · may be incorrectHardness 6.5–7 Mohs; brown, cream, and gray concentric or wavy bands; waxy to vitreous luster; cryptocrystalline quartz with no visible cleavage and conchoidal fracture. The smooth shape indicates mechanical tumbling and polishing.
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Physical properties
Hardness 6.5–7 Mohs; brown, cream, and gray concentric or wavy bands; waxy to vitreous luster; cryptocrystalline quartz with no visible cleavage and conchoidal fracture. The smooth shape indicates mechanical tumbling and polishing.
Formation & geological history
Forms when silica-rich groundwater fills cavities in volcanic rocks, depositing successive layers of chalcedony over long periods, commonly during the Cenozoic or older. Exact variety and locality cannot be confirmed from the photograph.
Uses & applications
Used for cabochons, beads, ornaments, paperweights, and inexpensive collector specimens; durable enough for some decorative stonework.
Geological facts
Banded agate is identified by contrasting chalcedony layers, often enhanced by natural iron or manganese staining. Common sources include Brazil, Uruguay, India, Madagascar, Mexico, and the United States.
Field identification & locations
Field identification: glassy/waxy luster, translucent edges, curved banding, hardness sufficient to scratch ordinary glass, and absence of cleavage. A streak test is white; acid causes no reaction.
