mineral

Black Onyx

Silicon Dioxide (SiO2)

Hardness: 6.5-7 (Mohs scale); Color: Jet black; Luster: Vitreous to silky; Crystal Structure: Trigonal (cryptocrystalline); Cleavage: None; Specific Gravity: 2.65-2.66

Hardness: 6
Color: Jet black
Luster: Vitreous to silky

Identified July 15, 2026More mineral →

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Physical properties

Hardness: 6.5-7 (Mohs scale); Color: Jet black; Luster: Vitreous to silky; Crystal Structure: Trigonal (cryptocrystalline); Cleavage: None; Specific Gravity: 2.65-2.66

Formation & geological history

Formed via the deposition of silica in gas cavities (vesicles) in lava, resulting in the distinctive bands or solid colors found in chalcedony. Geological age varies from tens to hundreds of millions of years.

Uses & applications

Primarily used in jewelry (rings, beads, intaglios), ornamental carvings, and as a popular stone for collectors and metaphysical practitioners.

Geological facts

While natural solid black onyx exists, much of the commercial black onyx sold today is actually grey chalcedony that has been dyed black using a sugar-sulfuric acid treatment, a process dating back to Roman times.

Field identification & locations

Identify by its smooth, waxy surface and lack of visible crystalline grain. It is often found in volcanic regions of Brazil, India, and Madagascar. In the field, look for opaque, dark nodules within basalt host rocks.