mineral

Black Onyx

Silicon Dioxide (SiO2) - Chalcedony variant

AI-generated identification · may be incorrect

Hardness: 6.5-7 (Mohs), Color: Solid black or black with white bands, Luster: Vitreous to silky when polished, Crystal structure: Trigonal/Monoclinic, Cleavage: None/Conchoidal fracture, Specific Gravity: 2.58-2.65

Hardness: 6

Identified May 25, 2026More mineral →

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Physical properties

Formation & geological history

Formed through the deposition of silica in gas cavities (vesicles) in lava or hydrothermal veins, often at low temperatures over millions of years. This piece appears to have been mechanically tumbled and polished.

Uses & applications

Primarily used in jewelry (rings, beads, intaglios), decorative carvings, and as a popular palm stone for mineral collectors.

Geological facts

True natural black onyx is rare; most black onyx sold commercially today is actually gray chalcedony that has been dyed or heat-treated with sugar-acid solutions to achieve a uniform jet-black color. It has been used since ancient times for amulets and seals.

Field identification & locations

Identify by its hardness (will scratch glass), heavy feel compared to plastic, and lack of visible grains. Commonly found in Brazil, India, Madagascar, and the USA. Collectors should look for evenness of color and high polish.