mineral

Black Onyx

Cryptocrystalline Chalcedony (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Black (often treated); Luster: Vitreous to waxy; Crystal Structure: Hexagonal (trigonal); Cleavage: None; Specific Gravity: 2.58-2.65

Hardness: 6
Color: Black (often treated)
Luster: Vitreous to waxy

Identified May 9, 2026More mineral →

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Physical properties

Hardness: 6.5-7 on Mohs scale; Color: Black (often treated); Luster: Vitreous to waxy; Crystal Structure: Hexagonal (trigonal); Cleavage: None; Specific Gravity: 2.58-2.65

Formation & geological history

Formed by the deposition of silica in gas cavities (vesicles) in lava, which results in the distinctive bands. Most solid black onyx found in jewelry is created by dyeing gray chalcedony.

Uses & applications

Primarily used in jewelry for cabochons, beads, and rings; also used for carvings, intaglios, and decorative ornaments.

Geological facts

Ancient Romans and Greeks used onyx for cameos and seal rings. The name comes from the Greek word 'onux' meaning fingernail or claw. Known as a protective stone in various cultures.

Field identification & locations

Identify by its smooth, waxy luster and lack of cleavage. In the field, it is found in volcanic regions. Collectors should look for evenness of color and high polish.