mineral

Black Onyx

Silicon Dioxide (SiO2) with cryptocrystalline structure

Hardness: 6.5-7 (Mohs scale); Color: Jet black with parallel banding (often treated); Luster: Vitreous to waxy; Crystal structure: Hexagonal/trigonal (microscopic); Cleavage: None; Specific gravity: 2.65-2.677

Hardness: 6
Color: Jet black with parallel banding (often treated)
Luster: Vitreous to waxy

Identified March 15, 2026More mineral →

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Physical properties

Formation & geological history

Onyx is a variety of chalcedony that forms in the gas cavities of lava. It is created through the slow deposition of silica from groundwater into volcanic rock pores. Most commercial black onyx is natural gray chalcedony that has been dyed black via sugar-acid treatment to achieve a uniform color.

Uses & applications

Primary use is in jewelry (cabochons, intaglios, beads) and ornamental carvings. Historically used for seals and cameo bases due to its layering properties.

Geological facts

In ancient Rome, Onyx was highly valued for seals because it was said the wax would not stick to it. While true onyx has parallel bands, 'black onyx' in the modern market is typically solid black chalcedony that has undergone a centuries-old dyeing process.

Field identification & locations

Identify by its high polish and lack of visible grains. In the field, look for volcanic basalt regions. It can be distinguished from black tourmaline by its lack of striations and from obsidian by its higher hardness and waxy luster.

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