Black Onyx

Physical Properties

Hardness: 6.5-7 (Mohs scale); Color: Jet black (often dyed); Luster: Vitreous/waxy; Crystal Structure: Trigonal (cryptocrystalline); Cleavage: None; Specific Gravity: 2.65-2.677.

Formation & Geological History

Onyx is a variety of chalcedony formed through the deposition of silica in gas cavities (vesicles) in volcanic lava. Over time, these silica layers form microcrystalline quartz bands. Most black onyx in the jewelry market is actually gray chalcedony that has been dyed black.

Uses & Applications

Primarily used in jewelry (rings, beads, pendants), ornamental carvings, cameos, and as a healing stone in lapidary arts.

Geological Facts

In ancient times, onyx was believed to protect against evil spirits. The term 'onyx' comes from the Greek word for 'fingernail' or 'claw.' While naturally solid black onyx exists, it is extremely rare; almost all commercial black onyx is treated to achieve its uniform color.

Field Identification & Locations

Identify in the field by its waxy luster, conchoidal fracture, and lack of visible crystal grains. It is often found in Brazil, India, Madagascar, and the USA. Collectors look for uniform opacity and high polish.

Identified on: 4/26/2026

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