mineral
Black Onyx
Silicon Dioxide (SiO2) with layered chalcedony
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)
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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.
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