mineral
Black Onyx
Cryptocrystalline Quartz (SiO2) with impurities
Hardness: 6.5-7 (Mohs scale), Color: Jet black, Luster: Vitreous to silky (when polished), Crystal Structure: Trigonal/Microcrystalline, Specific Gravity: 2.65-2.67
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs scale), Color: Jet black, Luster: Vitreous to silky (when polished), Crystal Structure: Trigonal/Microcrystalline, Specific Gravity: 2.65-2.67
Formation & geological history
Forms in the cavities of gas bubbles in volcanic lava. Over time, silica-rich fluids deposit layers, often creating the banded structure characteristic of chalcedony. Solid black onyx is frequently achieved through dyeing common chalcedony.
Uses & applications
Primarily used in jewelry for beads, cabochons, and faceted stones, as well as for decorative carvings, cameos, and intaglios.
Geological facts
In ancient times, onyx was believed to protect the wearer from the 'evil eye' and was often used in Roman seals because wax does not stick to it. Most black onyx sold today is actually gray chalcedony that has been soaked in a sugar solution and then carbonized in sulfuric acid.
Field identification & locations
Identified by its waxy-to-glassy luster when polished and its significant hardness. In the field, look for volcanic rock nodules. It is commonly found in Brazil, India, Madagascar, and the United States.
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