mineral

Black Onyx

Silicon Dioxide (SiO2) - variety of Chalcedony

Hardness: 6.5-7 Mohs; Color: Jet black (often dyed for uniform appearance); Luster: Vitreous to silky; Crystal Structure: Trigonal/Microcrystalline; Cleavage: None; Specific Gravity: 2.6-2.65

Hardness: 6
Color: Jet black (often dyed for uniform appearance)
Luster: Vitreous to silky

Identified June 13, 2026More mineral →

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Physical properties

Hardness: 6.5-7 Mohs; Color: Jet black (often dyed for uniform appearance); Luster: Vitreous to silky; Crystal Structure: Trigonal/Microcrystalline; Cleavage: None; Specific Gravity: 2.6-2.65

Formation & geological history

Formed in gas cavities (vesicles) of volcanic lava; silica-rich fluids deposit layers over thousands of years. Natural onyx often features parallel white and black bands, while solid black onyx is frequently heat-treated or carbon-dyed chalcedony.

Uses & applications

Primarily used in jewelry (rings, beads, cameos), ornamental carving, and as a decorative stone in fashion accessories.

Geological facts

In ancient Rome, onyx was widely used for seals because the wax would not stick to the stone. The name 'onyx' comes from the Greek word 'onux' meaning fingernail or claw.

Field identification & locations

Identified by its waxy luster, hardness (will scratch glass), and weight. Common locations include Brazil, India, Madagascar, and the USA. In the field, it appears as nodules or layers within volcanic rock units.

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