mineral

Black Onyx

Silicon Dioxide (SiO2) with layered chalcedony

AI-generated identification · may be incorrect

Hardness: 6.5-7 (Mohs); Color: solid black (often dyed); Luster: vitreous (waxy when raw); Crystal structure: trigonal (microcrystalline); Cleavage: none; Specific gravity: 2.65-2.667

Hardness: 6
Color: solid black (often dyed)
Luster: vitreous (waxy when raw)

Identified July 19, 2026More mineral →

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Physical properties

Hardness: 6.5-7 (Mohs); Color: solid black (often dyed); Luster: vitreous (waxy when raw); Crystal structure: trigonal (microcrystalline); Cleavage: none; Specific gravity: 2.65-2.667

Formation & geological history

Formed through the deposition of silica in gas cavities (vesicles) in volcanic lava. Over time, successive layers of chalcedony fill the voids, creating bands or solid colors.

Uses & applications

Primarily used in jewelry (rings, beads, pendants), ornamental carvings, and historical talismans.

Geological facts

Most solid black onyx on the market today is actually gray chalcedony that has been treated with a concentrated sugar solution and then carbonized in sulfuric acid to achieve the deep black color. This process has been used since ancient Roman times.

Field identification & locations

Identify by its high polish, lack of transparency (it is opaque), and hardness (it should not be scratched by a steel knife). Common sources include Brazil, India, Madagascar, and the USA.