mineral
Banded agate (likely black-and-white onyx)
Microcrystalline quartz, SiO₂; banded chalcedony
AI-generated identification · may be incorrectPolished, angular decorative specimen with alternating black and white bands. Quartz hardness is 6.5–7 Mohs; vitreous to waxy luster and no cleavage. The precise variety is difficult to confirm from the photo.
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Physical properties
Polished, angular decorative specimen with alternating black and white bands. Quartz hardness is 6.5–7 Mohs; vitreous to waxy luster and no cleavage. The precise variety is difficult to confirm from the photo.
Formation & geological history
Agate forms as silica-rich fluids deposit successive layers in cavities, often in volcanic rocks; banding records changes in fluid chemistry. Formation age varies with its host rock and cannot be determined visually.
Uses & applications
Used for carvings, ornaments, beads, cabochons, and occasionally architectural accents; this appears to be a small polished decorative piece.
Geological facts
“Onyx” commonly refers to chalcedony with parallel bands, while agate often has curved or irregular banding; commercial names can overlap.
Field identification & locations
Look for natural, tightly joined bands and a glassy-to-waxy polish; a scratch test is not recommended on a finished piece. Banded chalcedony occurs worldwide, including Brazil, Uruguay, India, and the United States.
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