mineral
Polished banded agate (likely sardonyx)
Cryptocrystalline chalcedony, SiO₂; reddish bands are iron-stained layers
AI-generated identification · may be incorrectMohs hardness 6.5–7; reddish-brown, white, and gray concentric banding; waxy to vitreous luster; microcrystalline quartz with no visible cleavage; specific gravity about 2.6.
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Physical properties
Mohs hardness 6.5–7; reddish-brown, white, and gray concentric banding; waxy to vitreous luster; microcrystalline quartz with no visible cleavage; specific gravity about 2.6.
Formation & geological history
Forms when silica-rich groundwater fills cavities in volcanic rock, depositing successive chalcedony layers; banding commonly developed during the Cenozoic, though agates occur in rocks of many ages.
Uses & applications
Used for cabochons, beads, carvings, paperweights, and decorative polished specimens; generally ornamental rather than industrial.
Geological facts
The rounded shape and glossy surface indicate tumbling or polishing. “Sardonyx” technically refers to layered red-brown and white chalcedony, while broader irregular banding is commonly marketed as banded agate.
Field identification & locations
Identify it by hardness, translucent edges, waxy luster, and curved continuous bands; it should not fizz in dilute acid. Common sources include Brazil, Uruguay, India, Madagascar, Mexico, and the western United States.
