gemstone

Polished black stone cabochon, possibly onyx

Likely chalcedony (microcrystalline SiO₂); exact material cannot be confirmed from the image

AI-generated identification · may be incorrect

Smooth, domed, opaque dark surface with a glossy polish; if onyx, Mohs hardness is about 6.5–7, with a trigonal microcrystalline structure and no visible cleavage. The photograph does not reveal diagnostic internal features.

Identified September 22, 2026More gemstone →

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Physical properties

Formation & geological history

Chalcedony forms as silica precipitates in cavities and fractures, commonly in volcanic rocks; banded onyx develops in successive silica layers over geologic time.

Uses & applications

Cut and polished as cabochons for jewelry, beads, and decorative objects; also collected as a lapidary specimen.

Geological facts

Black cabochons sold as onyx may instead be dyed chalcedony or other dark stones, so appearance alone is not definitive.

Field identification & locations

The domed, polished form and dark color fit a lapidary stone, but identification requires tests or examination of the unmounted material. Chalcedony occurs worldwide, including Brazil, India, Madagascar, and the United States.