Onyx (Black Onyx)

Physical Properties

Hardness: 6.5-7 (Mohs scale); Color: Black (often dyed), sometimes with white bands; Luster: Vitreous to silky; Crystal Structure: Hexagonal (Trigonal); Cleavage: None; Specific Gravity: 2.65-2.66

Formation & Geological History

Formed through the deposition of silica in gas cavities (vesicles) in lava, resulting in the distinctive bands found in many specimens. It is predominantly found in volcanic environments.

Uses & Applications

Primarily used in jewelry (rings, beads, cameos), ornamental carvings, and historical artifacts since ancient times.

Geological Facts

In ancient Rome, onyx was heavily used for seal rings because the wax would not stick to it. Most black onyx sold in jewelry today is actually heat-treated or stained gray chalcedony to ensure a uniform black color.

Field Identification & Locations

Identify by its waxy luster, lack of cleavage, and smooth conchoidal fracture. In the field, it is often found in Brazil, India, Madagascar, and the US (California/Arizona). Collectors look for natural un-dyed banding.

Identified on: 4/23/2026

Mode: Standard