Onyx (Black Onyx)

Physical Properties

Hardness: 6.5-7 on Mohs scale; Color: Solid black (often dyed); Luster: Vitreous to waxy; Crystal Structure: Trigonal/Microcrystalline; Cleavage: None; Specific Gravity: 2.65-2.67

Formation & Geological History

Onyx is a cryptocrystalline form of quartz (chalcedony). It forms in the gas cavities (vesicles) of volcanic lava, where silica-rich fluids deposit layers over thousands of years. It is found in igneous environments worldwide.

Uses & Applications

Predominantly used in jewelry (rings, beads, pendants), ornamental carvings, cameos, and intaglios. Historically used for seals and decorative architectural elements.

Geological Facts

Black onyx in the modern market is almost always natural grey chalcedony that has been treated with a sugar-acid solution or heat to achieve a uniform jet-black color. Ancient Greeks and Romans believed onyx could protect the wearer in battle.

Field Identification & Locations

Identify in the field by its smooth, conchoidal fracture and high hardness (cannot be scratched by a steel knife). Commonly found in Brazil, India, Madagascar, and the US. Most specimens in jewelry are identified by their opaque, uniform black finish.

Identified on: 4/17/2026

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