mineral

Black Onyx

Onyx (Silicon Dioxide, SiO2)

Hardness: 6.5-7 (Mohs); Color: solid jet black (often dyed for uniformity); Luster: vitreous; Crystal structure: hexagonal/trigonal (cryptocrystalline); Cleavage: none; Specific gravity: 2.65-2.677.

Hardness: 6
Color: solid jet black (often dyed for uniformity)
Luster: vitreous

Identified March 27, 2026More mineral →

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Physical properties

Hardness: 6.5-7 (Mohs); Color: solid jet black (often dyed for uniformity); Luster: vitreous; Crystal structure: hexagonal/trigonal (cryptocrystalline); Cleavage: none; Specific gravity: 2.65-2.677.

Formation & geological history

Formed through the deposition of silica in gas cavities (vesicles) in lava, commonly resulting in distinctive bands. Black onyx is a variety of chalcedony where the bands are parallel and often alternating with white.

Uses & applications

Predominantly used in jewelry (rings, beads, pendants), ornamental carvings, and historical intaglios. Also used in decorative home decor items.

Geological facts

In ancient times, onyx was believed to protect the wearer from 'evil eyes' and misfortune. Most solid black onyx sold today is actually gray chalcedony that has been treated with a sugar-acid solution to produce a permanent black color.

Field identification & locations

Identify by its waxy to vitreous luster and lack of visible grains. It is commonly found in Brazil, India, Madagascar, and the USA. In the field, look for nodules with a smooth, conchoidal fracture.