Black Onyx

Physical Properties

Hardness: 6.5-7 on Mohs scale; Color: Solid black (often treated); Luster: Vitreous to silky; Crystal Structure: Hexagonal/Trigonal (Cryptocrystalline); Cleavage: None; Specific Gravity: 2.65-2.667.

Formation & Geological History

Onyx is a variety of chalcedony that forms in gas cavities (vesicles) in volcanic lava. Over time, silica-rich fluids deposit layers, creating its banded or solid structure. It is found in igneous environments globally.

Uses & Applications

Primarily used in jewelry (rings, beads, cameos), ornamental carvings, and historical decorative arts. Historically used for seals and intaglio.

Geological Facts

Ancient Romans believed onyx gave them courage in battle. While natural black onyx exists, most commercial black onyx is actually gray chalcedony that has been dyed black naturally or artificially to achieve uniform color.

Field Identification & Locations

Identify by its waxy luster, lack of cleavage, and extreme hardness (will scratch glass). Commonly found in Brazil, India, Madagascar, and the USA. Collectors value uniform color and high-polish finishes.

Identified on: 4/21/2026

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